Change BETA banner for HMRC manuals

- Removed Slimmer beta label
- Added Slimmer template
- Included govuk_component template
- Added handling for different Beta Labels for HMRC and non HMRC manuals

Removed slimmer settings for beta banner and added in handling for different manuals (hmrc and non hmrc) to have different banner text

Trello ticket: https://trello.com/c/Z2z8wKgh/432-hmrc-manuals-frontend-3
Another minor point - you don't need `self` here. It's probably best to keep this consistent with the rest of the file.