update metadata sync gif

[sssoleileraaa]

Description

Fixes #1053

This is very subtle, but since we're playing a forever loop animation and stopping it once the metadata sync is finished, the stopping of the animation can look a bit abrupt. I fixed this by using nina's single loop animation of the metadata sync as the stopping animation rather than what we've been using: a gif of a static image.

Test Plan

Test with a small number of sources to ensure that every 15 seconds you see an animation pulse. Test with a large number of sources to ensure that that long metadata syncs show a continuous sync animation.

[sssoleileraaa]

Another thing to test is that the sync animation is continuous when the securedrop server cannot be reached and will stop once a sync succeeds

[eloquence]

Tested only in Docker env for now. Does what it says on the tin:

• Short syncs as short
• Long syncs are long (tested by adding sleeps to sync-related endpoints to Docker env)
• Failing syncs spin until connectivity is restored (tested by ^C-ing Docker env, restarting)

Normal sync (wait for it)

Loooong sync

Error state

[redshiftzero]

diff lgtm, thanks @creviera and thanks for functional testing @eloquence !