* Create "blank" EmailSubscriptionTag
* Refactor liquid_tags_used with spec
* Create /user_subscriptions#create
- Update liquid tag name to UserSubscriptionTag
* Add rate limiting and specs
* Add counter_culture for user_subscriptions
* Update /async_info/base_data
- Alphabetize user_data
- Add email
- Add subscription_source_article_ids
- Cache subscription_source_article_ids on User model
* Add stale email check and specs
* Change user_email to subscriber_email for clarity
* Restrict UserSubscriptionTag
* Rename RESTRICTED_TAGS to RESTRICTED_LIQUID_TAGS
* Make TODO comment more clear
* Refactor error responses and update specs
* Update type to source_type in error message
* Use constantize over safe_constantize
* Add check for active source
* Refactor checking of current_user's subscriptions
- Remove data from async_info
- Create a new service to fetch/cache a user's existing subscriptions
* Restrict email in base_data to admin roles
* Oops! Rename liquid tag file
* Change error back to result...oops!
* It's not goodbye, it's see you later. RIP email :/
* Add current_email to /user_subscriptions/base_data
* Revert adding current_email
* Undo async_info_controller changes/fix conflict
* Move params to constant
* Refactor SubscriptionCacheChecker
* Remove duplicate status code in JSON response
* Remove duplicate status code for #subscribed
* Use response.parsed_body
* Remove user guard in SubscriptionCacheChecker
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