refs https://github.com/TryGhost/Team/issues/559
Members controller was becoming bloated and difficult to follow due to catering for many different concerns.
- converted old modal to newer promise-modal style
- pulled full label-adding logic out of the members controller and into the modal so logic is contained in one place
- added `{{members-count-fetcher}}` resource that allows for member counts to be fetched directly from templates avoiding duplicated code
