Check if user is allowed to use Mozilla Sync

Adding methods to check if user is allowed to use Mozilla Sync. The
check is performed as a group membership test.
Register admin page and add template for user restriction
Add javascript and ajax files.
Add warning to personal page when user is not allowed to use Mozilla Sync
[tx-robot] updated from transifex
Add possibility to specify which group has access to Mozilla Sync.
Merge branch 'master' into mozilla_sync_1.2
Merge branch 'limit_to_group' into mozilla_sync_1.2

Give the admin the possibility to limit the usage of Mozilla Sync to
members of a certain group.

See issue #31.