- send email when subscription/trial marked for cancellation
- add new 'sub_cancel' email template
- send email with optional survey link, configured via 'survey_url'
- fixes #2229

Adds sending a cancellation email when a subscription is cancelled.
- The email may also include an option survey optional survey URL, if
configured in helm chart `survey_url` setting.
- Cancellation e-mail configured in `sub_cancel` e-mail template
- E-mails are sent to all org admins.
- Also adds `trialing_canceled` subscription state to differentiate from
a default `trialing` which will automatically rollover into `active`.
- The email is sent when: a new cancellation date is added for an
`active` subscription, or a `trialing` subscription is changed to to
`trialing_canceled`. (A subscription can be canceled/uncanceled several
times before actual date, and e-mail is sent every time it is canceled.)
- The 'You have X days left of your trial' is also always displayed when
state is in trialing_canceled.

Fixes #2229
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Co-authored-by: Tessa Walsh <[email protected]>