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/views/index.html

@@ -20,7 +20,7 @@
             {{template "views/partials/inprogress" .}}
             {{ if eq (len .ModelsConfig) 0 }}
             <h2 class="text-center text-3xl font-semibold text-gray-100"> <i class="text-yellow-200 ml-2 fa-solid fa-triangle-exclamation animate-pulse"></i> Ouch! seems you don't have any models installed from the LocalAI gallery!</h2>
-            <p class="text-center mt-4 text-xl">..install something from the <a class="text-gray-400 hover:text-white ml-1 px-3 py-2 rounded" href="/browse">🖼️ Gallery</a> or check the <a href="https://localai.io/basics/getting_started/" class="text-gray-400 hover:text-white ml-1 px-3 py-2 rounded"> <i class="fa-solid fa-book"></i> Getting started documentation </a></p>
+            <p class="text-center mt-4 text-xl">..install something from the <a class="text-gray-400 hover:text-white ml-1 px-3 py-2 rounded" href="browse">🖼️ Gallery</a> or check the <a href="https://localai.io/basics/getting_started/" class="text-gray-400 hover:text-white ml-1 px-3 py-2 rounded"> <i class="fa-solid fa-book"></i> Getting started documentation </a></p>
 
             {{ if ne (len .Models) 0 }}
             <hr class="my-4">
@@ -66,7 +66,7 @@
                         {{ end }}
                     </td>
                     <td class="px-4 py-3 font-bold">
-                        <p class="font-bold text-white flex items-center"><i class="fas fa-brain pr-2"></i><a href="/browse?term={{.Name}}">{{.Name}}</a></p>
+                        <p class="font-bold text-white flex items-center"><i class="fas fa-brain pr-2"></i><a href="browse?term={{.Name}}">{{.Name}}</a></p>
                     </td>
                     <td class="px-4 py-3 font-bold">
                         {{ if .Backend }}
@@ -84,7 +84,7 @@
                     <td class="px-4 py-3">
                         <button
                             class="float-right inline-block rounded bg-red-800 px-6 pb-2.5 mb-3 pt-2.5 text-xs font-medium uppercase leading-normal text-white shadow-primary-3 transition duration-150 ease-in-out hover:bg-red-accent-300 hover:shadow-red-2 focus:bg-red-accent-300 focus:shadow-primary-2 focus:outline-none focus:ring-0 active:bg-red-600 active:shadow-primary-2 dark:shadow-black/30 dark:hover:shadow-dark-strong dark:focus:shadow-dark-strong dark:active:shadow-dark-strong"
-                            data-twe-ripple-color="light" data-twe-ripple-init="" hx-confirm="Are you sure you wish to delete the model?" hx-post="/browse/delete/model/{{.Name}}" hx-swap="outerHTML"><i class="fa-solid fa-cancel pr-2"></i>Delete</button>
+                            data-twe-ripple-color="light" data-twe-ripple-init="" hx-confirm="Are you sure you wish to delete the model?" hx-post="browse/delete/model/{{.Name}}" hx-swap="outerHTML"><i class="fa-solid fa-cancel pr-2"></i>Delete</button>
                     </td>
                 {{ end }}
                 {{ range .Models }}