Remove the beta label from topic pages

We no longer consider topic pages to be in beta. This removes the
behaviour from `SpecialistSectorsController` which instructed Slimmer
to insert the beta notice into the page.
Use a custom beta message on the latest changes feed

Whilst we want to continue showing a beta label on the latest changes
feed, the beta label provided by the Slimmer processor (which says that
the page is 'being rebuilt') isn't accurate.

This replaces the use of the Slimmer beta notice processor with the use
of the govuk_component shared beta notice template, which allows us to
insert a custom message.