If a contributor is removed from a pledge, they should be notified of the change. This will send an email to any confirmed contributors letting them know.
* Rename generic "frontend" file to dialog
* Add a "remove pledge" action
* Process pledge deactivation
* Trigger an email on user-initiated pledge deactivation
* Show an error when trying to manage a deactivated pledge
* Add a label for "deactivated" pledges
* Split Auth functionality out to new file
* Move email-related code into the email file
* Use `assertFalse` for boolean assertions
* Add `can_manage_pledge` to check user or token against a given pledge
Pulled out of e9763f6678
* Remove duplicate test
This includes autofixes, along with some manual fixes; mostly syntax-related. Some `phpcs:ignore` comments were added for overzealous rules that don't apply.