Deprioritize background jobs over user-initiated actions

[eloquence]

To ensure the client is consistently responsive, we should always treat user-initiated actions (replies, starring, deletion, user-initiated sync) as higher priority than background operations (currently: automatic metadata syncs and jobs it adds to the queue).

Fully closing this issue is blocked on #655.

[eloquence]

WIP in branch: https://github.com/freedomofpress/securedrop-client/compare/deprioritize-downloads

[sssoleileraaa]

the only thing we shouldn't deprioritize is MetadataSyncJob but looks like @redshiftzero's WIP branch doesn't do that so this is fine

[eloquence]

Added to milestone; IMO must-do for beta as part of the critical work to optimize responsiveness of the client. See #652 and #657 as well.