Fix : allow use of subfolders in the url with reverse proxies

[anthonymasset]

This fix allow to use owc behind a reverse proxy using a subfolder in the url.
Ex : https://www.domain.fr/owc/

The index page generates all the paths correctly. t also works with urls without subfolders.
Ex : https://owc.domain.fr

[niccokunzmann]

Thanks for your fix! The tests show that the about page has a problem.. Could you check that we can visit the about page and also the about page works? The code should work with /index.html and / as well as /about.html and /calendar.html.

I think what you would like to achieve is to add everything before the first / to the DEFAULT_URL - is that what you would like to do?

[anthonymasset]

It should be ok now. The pytest fail because of the url tested that is absolute in place of relative. If I modify that on my computer, the test pass.

[niccokunzmann]

Thanks, this looks much more like it adresses the scale of the change - relative links!
I found that the tests fail because some absolute links are tested for. Here is an example:

link = f'<link rel="alternate" type="text/calendar" href="/calendar.ics?{html.escape(query)}" />'

[niccokunzmann]

It looks alright and we can merge when twe tests pass, I think.

[niccokunzmann]

Hi, can I help you with anything for this PR?