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
Split Topic Progres widget into its own controller, view, template
FEATURE: New expandable progress bar with jump buttons and jump to
arbitrary post.
Move up/down buttons on top of go
Add a way to validate min and max value of an integer site setting
FIX: lower per_page limits, and don't validate the pop3s_polling_username because that's NOT a discourse user
make autocomplete use highlight color for selecting
remove extra "go" from go top and go bottom
BUGFIX: not terminating self correctly on hangups from parent
SECURITY: XSS fix
BUGFIX: omit the "reply" word in mobile view
BUGFIX: mobile styling
BUGFIX: Don't depend on localStorage for Discourse to function
BUGFIX: broken images in edit history with markdown view
ES6: Remove an unncessary view
Version bump to v0.9.9.9