Switch daily emails to a digest format.

This code does two things:

One it makes the triage emails accept multiple issues
(modifying the old individual triage to send one issue as an array) and changes
the email to match tis. 

Two it adds a way to generate a list of users and batch their issues together
to send a daily digest (and switches the Rake task to use it).

If we decide that we want configurable emails then the digest generator stuff
will need modifying, we could prehaps use a flag on users to indiciate there
preferred format and then run two rake tasks but we should still be able to
use this as a base, as the email still makes sense for an individual issue.

I tested the emails locally using sendmail as I dont have permission to send
via sendgrid.