Use source user as "FROM" for emails

This gives certain user-triggered events
a "look" as though they came directly from
that user (the FROM and REPLY-TO headers
have the name of the source user).

This also introduces "letter_opener" into the
dev environment.  If you use "letter_opener"
as your send method for dev emails, it will
open all emails in a browser tab. Very useful.

closes: CNVS-11837

TEST PLAN:

Validate each of the following notifications
looks like it's "from" the source user:

- Submitting an assignment
- Resubmitting an assignment
- Posting to a discussion
- sending a conversation message
- posting a comment on a submission

Change-Id: I0e04aa9aa5bf31a590e69507e04b86b637247acf
Conflicts:
	app/models/message.rb
Reviewed-on: https://gerrit.instructure.com/35362
Reviewed-by: Drew Bowman <dbowman@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>