This is the top-level task for deploying [GlobalWatchlist](https://www.mediawiki.org/wiki/Extension:GlobalWatchlist), and extension developed as part of a [grant from the WMF](https://meta.wikimedia.org/wiki/Grants:Project/DannyS712/Create_a_global_watchlist_extension)
[x] Add the new extension submodule to the git mediawiki/extensions repo (4b2e5e5cc06e597e6b60c037257e0a5462516139)
[x] Add extension to the make-wmf-branch release tool (888d220b15b719e9a7074159a2fd6bd1df0b417a)
[x] Security review ({T260466})
[x] Move extension CI config to wikimedia-deployed section
[x] Deploy to beta cluster ({T268181})
[] Performance review ({T260860})
[] Set `$wmgUseGlobalWatchlist` to `true` for metawiki in `InitialiseSettings.php`
[] Update https://www.mediawiki.org/wiki/Extension:GlobalWatchlist ("Release status" etc)