show sync animation during the entire sync

[sssoleileraaa]

Description

Improve feedback indicator on first sync - right now users see the flash indicating that the sync started but we should at least keep flashing it until the sync completes (or something similar to flag to the user that the delay is expected). Tor is never going to be that snappy, and the sync may take a few seconds even when we remove keys from the get_sources endpoint. We can start this immediately.
(see #1009 (comment))

The motivation is to make it clearer to the user that the client is not frozen, especially during the first sync.