Verify more stuff is pre-filled on the form
Only show stats announcements tab where appropriate

This feature is going through some alpha testing by the ONS so we only
restrict visibility of the tab in the main menu to a subset of users.
Long term, this menu item is unlikely to feature so prominently, but
whilst they are testing the feature, it makes sense to keep it as
accessible as possible for them.
Fix typo
Correct statistics_announcements#destroy action

The main thing was to correct where the user is redirected to after
the record is destroyed. Also, it turns out the AR#destroy method
always returns the instance itself rather than a boolean value, so
there is no point switching on the return value.
Correct copy/paste fail

Was deleting the wrong thing!