Update beta notices on topics and latest changes feed

https://www.pivotaltracker.com/story/show/83834556

As they are no longer in 'beta', we're removing the beta label from the topic (aka 'specialist sector') pages. We're keeping the beta label on the latest changes feed, but we're changing the language so that it no longer suggests that it's being rebuilt.

This switches out the use of the Slimmer `BetaLabelInserter` in favour of using the new govuk_components functionality, which allows us to customise the message displayed.

/cc @edds @rboulton