Bug 1014994 - Part 1: Register RemoteTabsPanel. r=margaret

This is the boilerplate to add a new RemoteTabsPanel HomeFragment.  At
the moment, it merely displays a static view with a static string.

The panel title is "Synced Tabs", per yuan's design PDF; and the panel
appears at the front (far left) of the list on phones, and at the
back (far right) of the list on tablets.
Bug 1014994 - Part 2: Add RemoteTabsPanel configuration migration. r=margaret

This factors out a small migration helper, since we want the same logic
for Remote Tabs as existed for Recent Tabs.
Bug 1014994 - Part 3: Display static views in Remote Tabs panel when Account is missing or unhealthy. r=mcomella
Bug 1014994 - Part 4: Display list of Remote Tabs when possible. r=margaret
Bug 1014994 - Part 5: Enable swipe-to-refresh Remote Tabs list. r=margaret
Bug 1014994 - Part 6: Display device type icon. r=margaret
Bug 785199 - Add context menu to Remote Tabs home panel. r=margaret