Bug 836120 - Reduce memory overhead of Sync when it's not configured; r=rnewman

If Sync is (likely) not configured, the only loaded JS is for the XPCOM
service itself.

The UI code is now smart enough to initialize the Sync service if it
isn't yet loaded. This addresses bug 825728.
Bug 836177 - Temporarily merge more JSMs to mitigate compartment overhead; r=rnewman

As with bug 834936, this is meant to be temporary until zones land and
we have decent JSM overhead levels.