Sync icon should be disabled during active sync

[redshiftzero]

Double clicks on the sync icon seem to be triggering double syncs. The first click on the sync icon should disable the sync icon, preventing subsequent clicks from causing syncs (@creviera flagged to me that this is complicated by the fact that the disabled state is now the offline icon). Then the existing logic in RefreshButton._on_refresh_complete should update the icon back to active when the sync has completed.

[eloquence]

We discussed this briefly today. For the first iteration, we can keep the icon state as-is -- if the user clicks repeatedly, it'll still only add one job. We can revisit the use of a spinner or disabled state in future iterations.

[eloquence]

We will likely want to move away from an interactive refresh icon, see #655 (comment) and discussion in #343. Leaving open until that work is fully specified, but removing from milestone/backlog.