feat(ui): path prefix support via HTTP header

[mgoltzsche]

Description

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. 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 base URL using the BaseURL function into the HTML (head.html and login.html templates) 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.

This PR fixes #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 here.

Known limitations:

• gofiber swagger does not support executing API requests when a path prefix is used because it doesn't replace the base URL within the served swagger JSON, see Cannot try/execute endpoints within the swagger GUI when exposing it under a different path prefix via a reverse-proxy gofiber/swagger#104.
• gofiber swagger does not support using a web app with multiple path prefixes simultaneously, see Don't store the path prefix as global app state but derive it on a per-request basis gofiber/swagger#105.

Notes for Reviewers
Apart from the added automated tests, I did some manual testing and grepping, hoping to have all URLs covered.

Signed commits

• Yes, I signed my commits.

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[mudler]

Thank you 💯 - it makes totally sense as an addition to me. As soon as I have a bit of time will try to test out locally too to see if there aren't any regression

[mgoltzsche]

The app test failed. I think I fixed it now. Please re-run the workflow.

UPDATE: Also, I rebased the PR.