Implement bulk sync operation

Created by: malomarrec

User problem

On instances with many changesets, users want to get an updated view of some changesets before the next sync. Clicking on all changesets of interest to sync them is too manual / time consuming to be realistic in a medium to large batch change.

Workaround

Setup webhook. This is extremely unlikely to be used if users have webhook setup.

Proposal

Add a "sync" bulk action