Allow for more complex directory structures

[berolinux]

As of 1.10.0, gitea assumes all projects are in USERNAME/PROJECTNAME -- sometimes it would be useful to allow subdirectories in PROJECTNAME (e.g. when mirroring/forking the android source from https://android.googlesource.com/, to replicate the original structure, it would be nice to support repositories like USERNAME/device/generic/arm64 instead of having to replace upstream's directory structure altogether).

For this particular use case, it's probably sufficient to allow slashes in a project name.

[D3-jwatts]

FWIW, this is absolutely critical for my needs: I manage around 1200 repositories (30% are a single Android project), and without folder structures it would be impossible to navigate. :-(

Otherwise, we've been evaluating Gitea off/on for the last few weeks, and it is beautiful and manages every other requirement we have. Great work so far! 👍