Make AIO interface not dependent on absolute location

[Fly7113]

• close Add the possibility to host the AIO admin interface behind a reverse proxy (Nginx) but in a subfolder/subdirectory #5480

As a followup from #6782, here are the proposed changes to make AIO interface work relative to the browser location, without assuming the interface resides on the root path of the domain.
The changes can be summarised in:

• links in Twig templates: all the frontend links have been changed to be relative
• paths in location headers: (almost) all the location headers now redirect to the same page from where the request originated, which is (almost) always the "/container" page.
• getting the AIO path to inform Nextcloud of the correct location of the interface: a new frontend script collects and sends the path to the backend via the "start" button.

I tested every basic usage, including unauthenticated situations (which require a particular logic, as seen in the AuthMiddleware.php file) and modifying the settings.

[Fly7113]

Hm, I tired to squash the commits but it looks like I did something wrong... I hope this doesn't create confusion.

[szaimen]

First of all, thanks for creating this PR!

Hm, I tired to squash the commits but it looks like I did something wrong... I hope this doesn't create confusion.

Squashing is not necessarily needed but can you please rebase the branch from main? This would be important. Thanks in advance!

[Fly7113]

Rebased!
And squashed commits, so that the older and outdated commits (relative to my old work) are not visible. Less confusion!

[Fly7113]

Today, with the release of v11.8.0, I had the chance to test also the update process, and everything went smoothly!

[szaimen]

Thanks a lot for your effort!

Just to be clear: I try to review it in the coming 1-2 weeks and try to include it for the next major version which is planned to be released in ca 1 month.

[szaimen]

Hey, I finally had the time to have a look at this and have some comments below.

Have you already tested your changes btw and ensured that a default installation in a root dir still works?

[Fly7113]

Hello, thanks for finding the time to check it out.
I tested throughly in my "subfoldered" instance (daily driving it), and I also had the occasion to test it in a fresh environment without a base path.

I understand that the relative "Location" header can be confusing at times, especially since it's kind of a new feature: the HTTP spec has been updated in June 2014 to allow relative URIs in the "Location" header RFC 7231

Let me know what you think about the improved logic in the "Auth" middleware: I made the root scenario the first if case and added comments to explain the logic.