Remove all the occupation-related code and use core support wherever possible.
Bonus points for changing the string 'Taken over by Flow'.
/Title inspired by Erik's old patch for this: https://gerrit.wikimedia.org/r/#/c/160923/
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Deployment order:
When merging/deploying this, make sure to respect the order described in the commit messages:
1: https://gerrit.wikimedia.org/r/#/c/230370/ - ~~Populate rev_content_model (this script needs to be run on all Flow-enabled wikis in production before proceeding) (run: {T109671}).~~
2: https://gerrit.wikimedia.org/r/#/c/228267/ - ~~remove $wgFlowOccupyPages from Flow~~
3: https://gerrit.wikimedia.org/r/#/c/228271/ - ~~remove $wgFlowOccupyPages from config & prepare $wgNamespaceContentModels (in addition to $wgFlowOccupyNamespaces)~~
4: https://gerrit.wikimedia.org/r/#/c/228268/ - remove $wgFlowOccupyNamespaces from Flow, using $wgNamespaceContentModels instead
5: rest of the patches