Updated <MemberActivityEventParser> to handle email events
Updated <MemberActivityEventParser> to handle email events

refs https://github.com/TryGhost/Team/issues/1277

- if email events were passed through the parser they had blank actions and objects
- added `opened`, `received`, and `failed to receive` actions for email events
- changed object to equal `an email` for any email events

TryGhost/Ghost-Admin,TryGhost/Ghost-Admin