Plugin Directory Admin: Add 'button' or method to trigger plugin syncs via wp-admin

[Ipstenu]

In light of the June 18 outage where plugin updates aren't being properly synced, @otto42 pointed out it would be useful if the wp-admin page for a plugin had a button or such to 'force' an update. This would obviate the need for grabbing someone with systems access every time a plugin got stuck for whatever reason, and in cases like this would let us ensure security fixes were properly handled in a timely manner.

[dd32]

In 12166:

Plugin Directory: Add a button in wp-admin to trigger a plugin import from SVN.

Fixes #5277.