Replace occupation w/ ContentHandler integration.

Also converts all Flow actions into proper first-class MediaWiki
actions. Currently, these actions are used as a shim to the existing
Flow calling convention, but in a Future Refactor we can distribute
the Block-level actions into MediaWiki actions.

Moved from frontend-rewrite to master branch,
Original Change-Id: I8363489a11cdb57b8ae105d3d17e9cacc1c48ad3

Change-Id: I67f694a5926be5e8b66eda0891503eb30cf50165