Split up `repo.Action` in `routers/web` into smaller functions.
While some of the functionality was very similar (starring / watching),
they are ultimately separate actions. Rather than collecting all of them
under a single handler (`repo.Action`), split them up into smaller,
independent functions.
This does result in a little bit of code duplication, but the
independent functions should be easier to follow and understand.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>