Rename methods in Message Campaign

The methods to enroll and unenroll a user from a message campaign were
called 'startFor' and 'stopAll'.  This commit renames them to 'enroll' and 'unenroll'.
Additionally this commit changes the signature of 'unenroll' so instead of taking
a CampaignRequest it takes the only parameters it needs, externalId and campaignName.

Change-Id: I898738949c40e2435fec3db4df5a6ce0ba6e0b0d