* Add ProMembership model
* Implement ProMembershipsController#create
* Implement basic ProMembershipsController#show
* Add ProMembership to ledger
* Populate user history after pro subscription is created
* Add fields for expiration notifications
* Add ProMemberships::ExpirationNotifier to notify users of expiring memberships
* Add tasks for recurring jobs to notify users of expiration
* Add auto_recharge column to ProMembership
* Add ProMemberships::Biller (incomplete)
* Fix specs
* Add ProMembership to Administrate
* Fix spec
* Add has_enough_credits? to User and Organization
* Add Payments::Customer class
* Finish ProMembership::Biller functionality
* Fix ProMemberships::Creator check for credits
* Disable destroy actions for ProMembershipsController
* Correctly authenticate ProMembershipsController actions
* Make sure only pro user's history can be indexed
* Add ProMembershipsController#update action for auto recharge
* Use regular AR to save new credits and add touch to the purchaser
* Clarify Pro membership create policy
* Display information about an existing pro membership
* Add UI to show page
* Add system test for Pro membership creation
* Implement edit membership
* Make sure users with pro memberships can access history and dashboard pro
* Fix padding issue
* Show a different text for a user that has credits but not enough for Pro
* Move Pro Membership functionality inside settings
* Update Pro Membership link in email notifications
* Bust all relevant caches
* Add the Pro checkmark around the website
* Use Users::ResaveArticlesJob instead of delay
* Add/remove user from pro-members chat channel
* Use the appropriate Pro checkmark
* Remove unfinished pro elements
* Remove checkmark JS
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