Bug 869131 - [Email][Activity Indicator] Wrong use of spinner when setting up account. r=asuth

https://github.com/mozilla-b2g/gaia/pull/12505 with bitrot fixed and some
minor review cleanup fixes.  I've also done prepatory transition work for the
landing of https://bugzilla.mozilla.org/show_bug.cgi?id=940132 which will
implement anim-fade.  Some of the transition changes (like not specifying
'right') are primarily stylistic in nature, matching that bug.  Things will
work until the bug lands, they just won't fade in/out.
Merge pull request #14008 from asutherland/ac-spinner

Bug 869131 - [Email][Activity Indicator] Wrong use of spinner when setting up account. r=asuth