Extension diagnostics

[calpaterson]

Brief overview

AS A quarchive extension user

I WANT some basic actions and details on the extensions settings page

SO THAT I can figure out if my credentials are right, see when the last full sync was, etc

Additional details

• [x] ~Extension check correctness on save~
• [x] See last sync
• [x] Trigger a full sync
• [x] A full sync starts when the extension options are changed
• [x] Check should-sync first

[calpaterson]

Done