Merge branch 'feature/geo-improvements' into 'master'

Geo: Improvements and fixes after QA

We are going to move Geo (#76) to General Availability. This MR will handle all small fixes and changes to get a solid release

- [x] Replicates repository removal (using system hooks)
- [x] Replicates repository rename (using system hooks)
- [x] Replicates repository transfer (using system hooks)
- [x] Whitelisted Sidekiq routes so you can use web interface in a secondary node.
- [x] Change "Gitlab -> GitLab" for the readonly error flash
- [x] Check license add-on to allow any Geo features. 
- [x] Do not execute on a secondary node: `StuckCiBuildsWorker`
- [x] Do not execute on a secondary node: `HistoricalDataWorker`
- [x] Documentation changes: (master, slave -> primary, secondary)
- [x] Document we require `db_key_base`(secrets.yml) to be the same in all nodes

See merge request !354