Bug 940132 - [email] change drawers to slide down

The UX desire was to collapse folder_picker and account_picker into a drawer that slid down from under the header. This was accomplished by doing the following high level changes:

1. account_picker logic was folded into folder_picker, and account_picker removed.

2. The folder_picker is now an "immediate" card transition, but that card is initially transparent. The folder_picker then handles the transition of sliding down the folder list and fading in a background tap shield itself.

3. The "last sync" time now shows up in the message_list card in the bottom toolbar. It is the last sync time for that folder.

4. The main settings card now fades in on top of the folder_picker drawer.

5. Clicking on an account takes the user directly to the inbox for that account.

Some changes to fix jshint errors, but mostly in folder_picker, in an effort to inch towards jshint compliance.

Existing marionette tests already exercise the folder_picker (choosing a folder, switching accounts, opening settings) so no new tests with this change, just modifications to get the existing tests to pass with these changes.