Allow repo-centric and user-centric permissions sync to be enabled/disabled independently

Created by: mike-r-mclaughlin

Feature request description

Allow a site admin to enable/disable either repo-centric or user-centric permissions syncing.

Is your feature request related to a problem? If so, please describe.

See Slack discussion starting here.

From the above thread:

If we want to only rely on either user- or repo-centric syncing, we need to disable the other, because if they run simultaneously and yield inconsistent results (i.e. different list of repos due to token permissions), they'll keep overwriting each other (i.e. a user sometimes could see repos because user-centric syncing just finished, but sometime can't because repo-centric syncing just finished).