Set up the govuk_components functionality

Bumping Slimmer to 5.1, and including the `Slimmer::SharedTemplates` 
module in the `ApplicationController`, allows us to use the shared 
govuk_components for things like beta labels.
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.
Merge pull request #68 from alphagov/update-beta-notices

Update beta notices on topics and latest changes feed