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

Merge branch 'improve/3102-memoize-group-instance-variable-in-admin-groups-controller' into 'master'

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.

Added basic SQL guidelines

This is mostly based on the various changes I've made over the past few months.

See merge request !3091

update slack instructions

the previous instructions no longer apply, there is no "Incoming WebHooks" row anymore.

luckily there exists direct link to web hooks, so the previous steps aren't even neccessary

See merge request !3052

add link to web hooks docs

Add link to web hooks docs on custom hooks docs page.

See merge request !3082

adds language names to projects list [image attached]

See merge request !3000

[ci skip]

Deprecated GitLab CI API clean up

Deprecated GitLab CI API clean up.

The intent here is to clean up deprecated GitLab CI documentation leaving only relevant information.
Since we merged `Ci::Project` to `Project` most of this documentation is outdated.

Closes #13610 

See merge request !3003

add bundle clean to upgrade docs

This MR adds a bundle clean after installing new gems. Bundle clean removes old, not needed gems to keep the system clean.

See merge request !2809

Added Troubleshooting information for most used services.

This replaces the changes started at gitlab-com/www-gitlab-com!1603

I'd like to add similar troubleshooting information for EE only services, but that needs to happen on a different MR on the EE side. Probably after this one gets merged.

cc @ernstvn @dblessing @balameb @cabargas @kelvinmutuma 

See merge request !3080

Document changes to the initial admin password

Addresses https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3068#note_4117550

[ci skip]

See merge request !3120

[ci skip]

Remove redundant integration tests

These three tests were essentially checking that adding a note to
something updated its `noteable`'s `updated_at` attribute.

This is well-tested Rails behavior and we shouldn't feel the need to
write an integration test to verify it. At most we should be ensuring
that the association definition adds the `touch: true` option, which we
now do in Note's unit test.

See merge request !3117

Set cache headers for raw blobs

This changes allows browsers and (in the case of public projects)
proxy caches to cache raw Git blob responses.

See merge request !3113

[ci skip]

removes automatic setting of main_language to project for it to set a main language you have now to make a commit to the project