Prevent project team from being truncated too early during project destruction

There are two issues with truncating the project team early:

1. Projects::UnlinkForkService may not close merge requests properly since permissions may be revoked early.

2. If an error is encountered during flushing of caches, then the user will lose all privileges, possibly causing an issue on deletion on retry.

This came about after a review of possibly causes of project destruction issues in gitlab-com/support#413.

/cc: @jarka, @MrChrisW

Thank you @stanhu

Nice one, thanks @stanhu!

merged

EE merge is failing due to a conflict caused by Geo code in this worker. Should be easy to resolve.