Rework api/user/repos for pagination (#11827)

Rework api/user/repos for pagination (#11827)

* Add count to `GetUserRepositories` so that pagination can be supported for `/user/{username}/repos`
* Rework ListMyRepos to use models.SearchRepository

ListMyRepos was an odd one. It first fetched all user repositories and then tried to supplement them with accessible map. The end result was that:

* Limit for pagination did not work because accessible repos would always be appended
* The amount of pages was incorrect if one were to calculate it
* When paginating, all accessible repos would be shown on every page

Hopefully it should now work properly. Fixes #11800 and does not require any change on Drone-side as it can properly interpret and act on Link header which we now set.

Co-authored-by: Lauris BH <lauris@nix.lv>