Add ability to control sync on a per-repository basis

[jwir3]

Right now, the ability to sync (and my implementation of sync frequency) is on a whole agit-instance basis. This means that you can turn sync on or off for agit, but you can't turn it on or off for individual repositories. It would be nice to have more fine-grained control over these settings.

[kousu]

A bit off topic, but how do you control sync frequency? I installed Agit 1.38 from F-Droid and I don't see any option for this.

[jwir3]

@kousu Pull request #103 (issue #100) adds specifying the sync frequency via a menu. It hasn't been reviewed yet, so it's not in the code right now.