In MDL-67853, the "Enabled" label was added to all the toggle switches in the
user notification preferences page. This string is confusing, so this issue
is to remove it (and leave it as it was in 3.11).
Apart from that, following Helen's suggestion, I've renamed "Disallowed" to
"Locked off" and "Locked" to "Locked on", in order to help users to understand
better their meaning.
Replicates changes made to notification preferences in e41b3485.
The templates for each are similar enough than one can be used
from the other. Remove redundant styles for previous controls.
