Vector: Add navigation collapsing feature
- Moved from the Vector extension (removed there in I47950b5375cae38e2).
- Removed bucket testing
- Removed "new" version of collapsible nav (the language portal splitting feature) as it didn't seem to be active on any wikis. Can be revived later if considered useful still.
- Killed the vector-collapsiblenav preference
- Usage on Wikimedia wikis:
- 0.008% on en.wikipedia
- 0.02% on de.wikipedia
- Usage on Wikimedia wikis:
Bug: 46512
Change-Id: I87149d7e15931f02ab700164e9e1d3d707b5e6a5