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/utils/baseurl_test.go

@@ -0,0 +1,48 @@
+package utils
+
+import (
+	"net/http/httptest"
+	"testing"
+
+	"github.com/gofiber/fiber/v2"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBaseURL(t *testing.T) {
+	for _, tc := range []struct {
+		name      string
+		prefix    string
+		expectURL string
+	}{
+		{
+			name:      "without prefix",
+			prefix:    "/",
+			expectURL: "http://example.com/",
+		},
+		{
+			name:      "with prefix",
+			prefix:    "/myprefix/",
+			expectURL: "http://example.com/myprefix/",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			app := fiber.New()
+			actualURL := ""
+
+			app.Get(tc.prefix+"hello/world", func(c *fiber.Ctx) error {
+				if tc.prefix != "/" {
+					c.Path("/hello/world")
+				}
+				actualURL = BaseURL(c)
+				return nil
+			})
+
+			req := httptest.NewRequest("GET", tc.prefix+"hello/world", nil)
+			resp, err := app.Test(req, -1)
+
+			require.NoError(t, err)
+			require.Equal(t, 200, resp.StatusCode, "response status code")
+			require.Equal(t, tc.expectURL, actualURL, "base URL")
+		})
+	}
+}