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It will not appear in `rake -T` output, and the dependent gems are not
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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

Expose User#last_sign_in_at and User#confirmed_at for admins

Closes #840

See merge request !3090

Remove instance variable `@group` multiple assignment

See merge request !3127

Remove duplicate 'cache_store' configuration in test environment

[ci skip]

See merge request !3061

This ensures the right caches are flushed when adding a branch via the
UI, instead of only flushing this one random cache.

This ensures Repository#add_tag calls Repository#before_push_tag instead
of just 1 random cache expiration method.

This ensures that Repository#rm_branch calls
before_remove_branch/after_remove_branch instead of just 1 random cache
expiration method.

The methods used for this are Repository#tag_count and
Repository#branch_count which cache their output in Redis as well as
memoizing it in an instance variable. Both methods have a corresponding
methods/hooks to flush the caches at the right time.