use new email template when joining a group plan for customisation of subscription cancellation information (#8637)

* 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

test/api/v3/unit/libs/stripePayments.test.js

@@ -683,6 +683,7 @@ describe('Stripe Payments', () => {
           groupId: undefined,
           nextBill: currentPeriodEndTimeStamp * 1000, // timestamp in seconds
           paymentMethod: 'Stripe',
+          cancellationReason: undefined,
         });
       });
 
@@ -702,6 +703,7 @@ describe('Stripe Payments', () => {
           groupId,
           nextBill: currentPeriodEndTimeStamp * 1000, // timestamp in seconds
           paymentMethod: 'Stripe',
+          cancellationReason: undefined,
         });
       });
     });