* use new email template when subscription is cancelled from joining a group plan
* use new email template when subscription is cancelled from joining a group plan - needs more code, tests
* change from sending new email as a cancel-subscription option to sending as a group plan join email
Uses a new group-member-join email template instead of old group-member-joining because new template includes mandril conditional merge tags.
Also adds tests and comments. Edits some comments for accuracy and typo fixes.
* adapt group-member-join email template for manual cancel message for iOS and Android subscriptions
* save test user so its profile name can be read by calls to sendTxn
* add documentation for the user model cancelSubscription function
* add constants for strings passed to mandrill email templates