Support for insecure POP3 (without SSL)

Ideally it would be a menu selection to select POP3, POP3S, and potentially other future protocols like IMAP if desired, but I didn't want to deal with data migration at this point.  And then I was going to have a checkbox for "Secure" (on by default, obviously), but that was very hard to word as to how it was different given everything else referred to pop3s and I couldn't change that either.  So I settled on a preference:

pop3s_polling_insecure: "Poll using plain text POP3 without SSL"

Off by default.

This makes it very clear that as to what turning on that checkbox will be, and by calling it "insecure" makes sure people will think twice before turning it on.

I have not attempted to do any of the translations of the preference, I'm ot sure how you handle that.
Bugfix: Mailing list mode disables PM emails

Emailing a PM uses the user_posted event/format, but Mailing List Mode disables emails with this format.

We work around this by temporarily using "postedprivate", and then reverting back to "posted" efter the Mailing List Mode check.
Add custom embed_by_username feature

Feature to allow each imported post to be created using a different discourse
username. A possible use case of this is a multi-author blog where discourse
is being used to track comments. This feature allows authors to receive
updates when someone leaves a comment on one of their articles because each of
the imported posts can be created using the discourse username of the author.
Bugfix: Mailing list mode disables PM emails (take 2)

Mailing List Mode disables normal emails, but mailing list mode does not email private_messages.

To avoid this, mailing list mode should only disable emails if they are not private_messages.

There are no tests for this commit as UserNotifications does not appear to be instantiated in the spec suite.
update heroku install docs

1. Redis requires password
2. Automatic asset compilation on by default
3. Tell rails to serve compiled assets
4. Increase garbage collection limit
5. Link to S3 instructions
Highlight posts when jumping to them.
Keyboard shortcuts for new progress expansion
Merge pull request #2441 from adamloving/update-heroku-docs

update heroku install docs
Merge pull request #2439 from peternlewis/mailing_list_mode_disables_private_message_emails2

Bugfix: Mailing list mode disables PM emails (take 2)
Make it possible to add multiple CORS origins in discourse.conf and docker yml files
move bio to UserProfile from User
update import script to save bio to UserProfile
FIX: crawler versions of the page need the updated bio relationships.
Add tooltip to Admin & Moderator glyph
Merge pull request #2442 from techAPJ/add-tooltip-to-glyph

Add tooltip to Admin & Moderator glyph
Extra safety to the post stream. Don't return `undefined` if we can't
retrieve a post for some reason.
Don't disable the Sign In button in the sign in modal
FIX: Remove extra setting of `view`
FIX: Replace protocol relative URLs in emails
some email notification header fixes