Replace refresh icon with non-interactive sync activity indicator

[eloquence]

• Blocked by Separate MetadataSyncJob from main queue and increase frequency of background syncs #652, though a preliminary pass may be included as part of it.
• Blocked by UX work described in Update Text/Icon/Behavior of "Sync button" area securedrop-ux#97

Since we are planning to shift from on-demand syncs to continuous syncs, it makes no sense for the refresh widget to be interactive. Instead, we just want to let the user know the following:

1. whether they're online or offline;
2. whether they're currently receiving new messages/replies;
3. (if they're offline, or network operations are paused) when the last refresh has taken place.

[ninavizz]

Item N°2 from above has yet to be spec'd on the UX front, but items N°1 and N°3 have been added to the UX issue. Will post here when N°2 has been posted.

[eloquence]

See freedomofpress/securedrop-ux#97 (comment) for proposed prioritization.

[sssoleileraaa]

I'm beginning work on this one