Up Sidekiq Concurrency, pin to DB pool amount.

[dandrabik]

WHAT

Given we have an increase in background job volume, I want to experiment with upping the number of workers per server. Trying 8 workers up from 4.

WHY

More jobs, more problems.

HOW

1. Pin the sidekiq concurrency to the Rails Thread Max which is used to set the number of DB connections (so every thread gets a Db connection and we don't get errors for no Connection available).
2. Pass the thread count in an ENV variable in the work Procfile used by Heroku so the BG workers can have a different thread count than the Web servers.

Mainly getting this from here: sidekiq/sidekiq#2985 (comment)

Screenshots

PR Checklist	Your Answer
Have you added and/or updated tests?	NO, not really testable
Have you deployed to Staging?	Not yet - deploying now!
Self-Review: Have you done an initial self-review of the code below on Github?	Yes
Design Review: If applicable, have you compared the coded design to the mockups?	N/A

[dandrabik]

Ok, Looking at the memory graphs, I don't think we should add more workers per dyno (We are at max memory usage), instead we should just add more dynos.