Tidy up how we concatenate host and path.

props @westi

Merges r96399-wpcom.
Merges r96400-wpcom.
REST API: versioning support for REST API endpoint

Props @jonathansadowski @jkudish

Merges r96274-wpcom.
Merges r96288-wpcom.
Merges r96289-wpcom.
Merge pull request #490 from Automattic/wpcom-merge-april-23-george

Merge a couple changes in from wpcom