Standardise casing of "Service Explorer" strapline

Because we only allow certain options for strapline in stagecraft, we
should standardise where these are used.
Merge pull request #793 from alphagov/make-service-explorer-consistent

Standardise casing of "Service Explorer" strapline
Move hmrc-prototypes to top level to resolve slug issues

- Migration would currently incorrectly migrated hmrc-prototypes/something
- The slug now indicates that the dashboard is a prototype
Merge pull request #795 from alphagov/rename_hmrc_prototypes

Move hmrc-prototypes to top level to resolve slug issues