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.