Add support for reverse proxy path prefixes

[jk1ng]

Support for parsing the public_url parameter for the path prefix is now in-place.

[jk1ng]

Whoops, I didn't realize this was going to do a PR against the upstream repository; rookie mistake.

Feel encouraged to close this with prejudice - alternatively, happy to work with your team to adjust this code to be suitable for merging.

One problem with this kind of PR is that the front end JavaScript is minified so it's not trivial to see what changed. The only things I should have changed there were: 1) adding support for reading the new path prefix HTML Element dataset property (added to src/textual_serve/templates/app_index.html) by adding this.pathPrefix=e.dataset.pathPrefix; to the w constructor, and 2) adding support for using that value later in ${window.location.origin}/${this.pathPrefix}download/${i}${r} (the URL the browser is directed to open when downloading a file).