Ability to delete/hide subscriptions

[ymilki]

In line with #46, we need a way to delete existing subscriptions.

We currently want to consolidate existing subscription times into one time, and even completely delete unused subscriptions.

The challenge with deleting existing subscriptions is handling all the references to subscriptions. Simply deleting the subscription will leave dangling references in the database. In turn, users that were signed up for the subscription will continue receiving emails for the subscription.

[ymilki]

Alternatively, making subscriptions active/inactive would be sufficient.