993 - Email grace period

Introduces a grace period for emails.
When an email activity stream contains an activity that was delivered in
the last `config.activity.mail.gracePeriod` seconds, we will not deliver
email for that stream and postpone it till the next collection.
Email grace period

Bump polling frequency to 15 minutes and grace period to 3 minutes.
Also reworded some comments in the email collector.
Fixed grace period documentation error