Bug Report: Absolute path on assets and links

Describe the bug

If the subspace instance is running behind a reverse proxy under a subdirectory, such as https://example.org/subspace/, the application will attempt to source the static assets from https://example.org/static/... instead of https://example.org/subspace/static/..., links and buttons will also goes to the root directory(https://example.org/logout) as well.

This is due to the hard coded absolute path inside the web templates, and can be rectified by prepending the SUBSPACE_BACKLINK variable value back into the web template.

Note: The current "Subspace" text, which sits next to the home icon on the navigation bar, is a great example demonstrating the hard coded absolute path issue.

Proposed solution

Referencing to this line: https://github.com/subspacecommunity/subspace/blob/32fd13b07b9279e095782c3c198b0088fd184380/web/templates/header.html#L25

And also these lines with absolute path issues: https://github.com/subspacecommunity/subspace/blob/32fd13b07b9279e095782c3c198b0088fd184380/web/templates/header.html#L8-L18

It can be resolved by simply prepending the backlink template variable, for example

        <link rel="icon" href="{{.Backlink}}/static/favicon.png?v={{$.Version}}">
        <link rel="apple-touch-icon" href="{{.Backlink}}/static/favicon.png?v={{$.Version}}">

        <title>Subspace</title>

        <link rel="stylesheet" type="text/css" href="{{.Backlink}}/static/semantic/semantic.min.css?v={{$.Version}}">
        <link rel="stylesheet" type="text/css" href="{{.Backlink}}/static/style.css?v={{$.Version}}">
        <link rel="stylesheet" type="text/css" href="{{.Backlink}}/static/roboto.css?v={{$.Version}}">

        <script src="{{.Backlink}}/static/jquery.min.js?v={{$.Version}}"></script>
        <script src="{{.Backlink}}/static/semantic/semantic.min.js?v={{$.Version}}"></script>

To Reproduce

Steps to reproduce the behavior:

Run a subspace instance
Reverse proxy it behind nginx/swag/traefik/etc
Browse the corresponding URL of the web server, which is a proxy to the subspace instance
See the errors happen inside the browser console/inspector

Expected behavior

The assets and navigation links/buttons should be prepended with the backlink variable value.

Screenshots

subspace-reverseproxy-absolutepath-issue

Additional context

Sorry that I don't have time to make a PR at the very moment. However, if this issue isn't getting a fix within a week, I might squeeze some time to fix it.

subspacecommunity / subspace