Improve Email Sending Resilience

[rhysyngsun]

We've stepped up the scale of email we send so we probably want to improve the performance at which we send emails. Capturing some issues we noticed:

• There's some kind of slow memory leak in sending frontpage emails (Investigate memory leak #1983)
• The notifications.tasks.send_weekly_frontpage_digests task takes a while to complete. We probably need a fan-out structure here
• Correctly size and possibly dedicate a worker to email sending. Currently pending emails and various other tasks pile up behind the queuing.

[pdpinch]

@rhysyngsun do you think we can close this. I think you addressed the 2nd and 3rd point and the first point has its own issue.

[rhysyngsun]

@pdpinch it could still use some work to avoid bottlenecking effects (bullet point 3)