Slack application service

See merge request !2259

Update Geo documentation to use PostgreSQL replication slots

Closes #1844

See merge request !2245

Fix EE conflicts for 'Fix `migration:path` build for "Honor the 'Remember me' parameter for OAuth-based login"'

See merge request !2355

Update alpha database documentation to reflect change in attribute

See merge request !2252

Improve state management for deploy boards with realtime data

Closes #2677

See merge request !2311

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