Use dedicated methods for counting branches and tags

This started out as "Lets add two methods to count and cache some data" and ended up in a clean-up/fix of some existing code. The two problems were:

1. Different code was used for adding/removing branches/tags via Git and the UI
2. The code used for the UI didn't have any RSpec tests, and I couldn't find any Spinach tests either (though grepping for Spinach stuff is hard)

This MR addresses the following:

1. `Repository#branch_count` and `Repository#tag_count` are used to count and cache the number of branches/tags, these methods are then used on the branches/commits/tags pages.
2. `Repository#add_tag`, `Repository#add_branch`, `Repository#rm_tag` and `Repository#rm_branch` now all the appropriate before/after hook methods instead of calling a random single cache expiration method. This ensures caches are properly flushed when adding/removing tags/branches via the UI.
3. RSpec tests were added for the above methods.

This fixes gitlab-org/gitlab-ce#13459

See merge request !3128