Put notificiations behind a feature flag

And only show the notifications panel for account settings when it is turned on

Fix #1762
Add renderer_global to expose feature to templates
Split settings tabs; make auth and settings blocks

Make the auth and settings sections of the app template into blocks so
that they can easily be overridden in inheriting templates. That makes
it possible to split the account and notifications settings into their
own templates without worrying about whether they are applicable for
downstream partners, who can now override the whole block rather than
the account template, for instance.
Merge pull request #1811 from hypothesis/notification-feature-flag

Put notifications behind a feature flag