Send separate nightly email for comments

[andrewbiang888]

This PR pulls out comment activity into a separate email to make it easier for city/campaigns to parse. This PR follows the existing pattern of having the city be cc'd on campaign emails. Should be send two emails instead?

Ref: #1131

[jaronheard]

I think that following the same pattern as the activity emails makes sense for now.

[jaronheard]

As a note, a couple of potential items to follow up on:

• link comment to associated transaction by id (this is a little tricky because the activity log doesn't have a transaction id, but should be somewhat low-hanging fruit)
• maybe someday we should make previewing emails in development easier, but this is not low-hanging fruit