EE: Add foreign keys to various tables that point to the "projects" table

See merge request !2223

Add overview and use cases for db load balancing docs

See merge request !2342

This cop has been renamed to ActiveRecordSerialize to match the way
"ActiveRecord" is usually written.

These are specific to EE and a whole bunch of these can be fixed by just
removing the `dependent:` option since we now have more foreign keys in
place.

This adds various missing foreign keys (with cascading deletes) that
either point directly or indirectly to the "projects" table.

These IDs point to non-existing rows, causing the foreign key
constraints to fail.

This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

This removes the need for relying on Rails' "dependent" option for data
removal, which is _incredibly_ slow (even when using :delete_all) when
deleting large amounts of data. This also ensures data consistency is
enforced on DB level and not on application level (something Rails is
really bad at).

This commit also includes various migrations to add foreign keys to
tables that eventually point to "projects" to ensure no rows get
orphaned upon removing a project.

Add use cases for issues-related EE features

Closes #2810

See merge request !2330

Add docs for Pivotal tile installation

See merge request !2341

Update Auditor users docs

See merge request !2333

[EE] Create and use project path helpers that only need a project, no namespace

See merge request !2336

[EE] Disable RSpec/BeforeAfterAll and enable RSpec/ImplicitExpect cops

See merge request !2305

Geo: Migrated checks to SystemChecks

See merge request !2331

[ci skip]

[ci skip]

Use trigger(:click) instead of trigger to avoid a weird transient bug

Closes #2843

See merge request !2340

Environment-specific variables

Closes #2302

See merge request !2112

Revert "Merge branch '18000-remember-me-for-oauth-login-ee' into 'master'"

See merge request !2345

Add a license check for group-webhooks

Closes #2576

See merge request !2280

This reverts merge request !2175

Fix spec failure for squash in progress error handling

See merge request !2343

Fixed admin sidebar not showing all options in new navigation

See merge request !2334

Port of 32838-admin-panel-spacing to EE

See merge request !2264

Show loading icon when retrieving Geo node status

Closes #1977

See merge request !2309

Resolve EE conflicts for "Fix API Scoping"

See merge request !2338

Similar to how we check project features.

- Hide the `webhooks` link from the group-settings page
- All group-webhooks-pages render a 404
- Don't execute webhooks if the feature is disabled

EE Port: Honor the "Remember me" parameter for OAuth-based login

See merge request !2175