Bug 946101 - update syncRange when account updated, tests for it. r=jrburke

land https://github.com/mozilla-b2g/gaia-email-libs-and-more/pull/349
To make sure we update syncRange in the front-end objects so the UI updates
without requiring the user to close the settings UI and reopen it.
Merge pull request #25903 from asutherland/modifyaccount-syncrange

Bug 946101 - update syncRange when account updated, tests for it. r=jrburke