Fixes to MYSQL queries driving Subscription Stats page

[17sushmita]

Part of larger planning issue #9712

Fixes to MYSQL queries driving https://publiclab.org/stats/subscriptions , which has known bugs at #7908

• Change heading from ‘People who have posted’ to 'People who have contributed' on contributors section of tag information page (FTO)
• Fix MySQL queries on subscriptions page (access to the database of unstable version may be required)
• Write Functional tests

[jywarren]

Fixed in #9828!