Add mysql db

[mashehu]

Moving the backend for the stats page to a MySQL database (as proposed in #570).

Tables the database should have:

• github org members (fields are username, github_id, github_url and date added to database (I can't find the option to get the date they were added to the github org)
• github commits (including merge) (commit sha, author (have to filter specifically for merges), commit date, lines added, lines removed)
• Twitter followers
• Slack users
• ...anything else in the /api/cache folder? If it's JSON files anyway..

[ewels]

Can add db logins to this file: https://github.com/nf-core/nf-co.re/blob/master/config_example.ini

[mashehu]

Replaced by #929.