Immediately refresh viewer for live-updates

Our applyUpdates() puts/deletes annotations into the viewer.
But for the stream page, because there was no scope digest cycle
it seemed that the UI was reacting very slow to show the annotations.

This one-liner just creates a digest cycle in these scenarios.
The user experience is a magnitude better with this in the stream page

Fix #1455
Merge pull request #1461 from hypothesis/1455-refresh-viewer-immediately

Immediately refresh viewer for live-updates
Remove unused variables
Introduce user multifield

In the annotation.user field we store the users in the acct:username@domain format
However when we're searching for users via stream we only use the username part
and the client has to juggle to make up the domain and the backend
cannot theoretically know the domain.

So, the user field was turned into a multi-field, to reflect both usage.
annotation.user is the same as it was and
annotation.username contains only the username part.

Now the client doesn't have to construct back the whole user format.
And without this our 'any' fields searches failed for the user field

Fix #1456
Fix #1457