New Block: display subs generated for group content

Problem/Motivation

Group moderators or admins may benefit from seeing how many emails were generated for a given piece of content. We can hook into the subscription_builder process to record a count for how many immediate and daily subscriptions were created from each piece of content. That data can then be queried and displayed.

Proposed resolution

Create a database table to track subscriptions generated for a piece of content. Query and display this data in a block which can be displayed on group content pages. This should only be visible to group members with the view group subscription statistics permission.

Later we will further enhance this block to display a count for views generated from email.

Data model changes

We will need a new database table to track this data. To be forward thinking we should have a column specifying when the subs were generated: on initial save, or on a future edit. I do plan to update the group_subscription module in the future with the ability to generate new subs when content is modified.