feat(ui): path prefix support via HTTP header (#4497)

Makes the web app honour the `X-Forwarded-Prefix` HTTP request header that may be sent by a reverse-proxy in order to inform the app that its public routes contain a path prefix.
For instance this allows to serve the webapp via a reverse-proxy/ingress controller under a path prefix/sub path such as e.g. `/localai/` while still being able to use the regular LocalAI routes/paths without prefix when directly connecting to the LocalAI server.

Changes:
* Add new `StripPathPrefix` middleware to strip the path prefix (provided with the `X-Forwarded-Prefix` HTTP request header) from the request path prior to matching the HTTP route.
* Add a `BaseURL` utility function to build the base URL, honouring the `X-Forwarded-Prefix` HTTP request header.
* Generate the derived base URL into the HTML (`head.html` template) as `<base/>` tag.
* Make all webapp-internal URLs (within HTML+JS) relative in order to make the browser resolve them against the `<base/>` URL specified within each HTML page's header.
* Make font URLs within the CSS files relative to the CSS file.
* Generate redirect location URLs using the new `BaseURL` function.
* Use the new `BaseURL` function to generate absolute URLs within gallery JSON responses.

Closes #3095

TL;DR:
The header-based approach allows to move the path prefix configuration concern completely to the reverse-proxy/ingress as opposed to having to align the path prefix configuration between LocalAI, the reverse-proxy and potentially other internal LocalAI clients.
The gofiber swagger handler already supports path prefixes this way, see e2d9e9916d/swagger.go (L79)

Signed-off-by: Max Goltzsche <max.goltzsche@gmail.com>

core/http/middleware/strippathprefix_test.go

@@ -0,0 +1,121 @@
+package middleware
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gofiber/fiber/v2"
+	"github.com/stretchr/testify/require"
+)
+
+func TestStripPathPrefix(t *testing.T) {
+	var actualPath string
+
+	app := fiber.New()
+
+	app.Use(StripPathPrefix())
+
+	app.Get("/hello/world", func(c *fiber.Ctx) error {
+		actualPath = c.Path()
+		return nil
+	})
+
+	app.Get("/", func(c *fiber.Ctx) error {
+		actualPath = c.Path()
+		return nil
+	})
+
+	for _, tc := range []struct {
+		name         string
+		path         string
+		prefixHeader []string
+		expectStatus int
+		expectPath   string
+	}{
+		{
+			name:         "without prefix and header",
+			path:         "/hello/world",
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "without prefix and headers on root path",
+			path:         "/",
+			expectStatus: 200,
+			expectPath:   "/",
+		},
+		{
+			name:         "without prefix but header",
+			path:         "/hello/world",
+			prefixHeader: []string{"/otherprefix/"},
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "with prefix but non-matching header",
+			path:         "/prefix/hello/world",
+			prefixHeader: []string{"/otherprefix/"},
+			expectStatus: 404,
+		},
+		{
+			name:         "with prefix and matching header",
+			path:         "/myprefix/hello/world",
+			prefixHeader: []string{"/myprefix/"},
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "with prefix and 1st header matching",
+			path:         "/myprefix/hello/world",
+			prefixHeader: []string{"/myprefix/", "/otherprefix/"},
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "with prefix and 2nd header matching",
+			path:         "/myprefix/hello/world",
+			prefixHeader: []string{"/otherprefix/", "/myprefix/"},
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "with prefix and header not ending with slash",
+			path:         "/myprefix/hello/world",
+			prefixHeader: []string{"/myprefix"},
+			expectStatus: 200,
+			expectPath:   "/hello/world",
+		},
+		{
+			name:         "with prefix and non-matching header not ending with slash",
+			path:         "/myprefix-suffix/hello/world",
+			prefixHeader: []string{"/myprefix"},
+			expectStatus: 404,
+		},
+		{
+			name:         "redirect when prefix does not end with a slash",
+			path:         "/myprefix",
+			prefixHeader: []string{"/myprefix"},
+			expectStatus: 302,
+			expectPath:   "/myprefix/",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			actualPath = ""
+			req := httptest.NewRequest("GET", tc.path, nil)
+			if tc.prefixHeader != nil {
+				req.Header["X-Forwarded-Prefix"] = tc.prefixHeader
+			}
+
+			resp, err := app.Test(req, -1)
+
+			require.NoError(t, err)
+			require.Equal(t, tc.expectStatus, resp.StatusCode, "response status code")
+
+			if tc.expectStatus == 200 {
+				require.Equal(t, tc.expectPath, actualPath, "rewritten path")
+			} else if tc.expectStatus == 302 {
+				require.Equal(t, tc.expectPath, resp.Header.Get("Location"), "redirect location")
+			}
+		})
+	}
+}