Fixes #7681

closes #7682

closes #7624

fixes #1649

closes #7124

closes #7050

closes #6817

* Add markerb to the Gemfile
* Update mailer classes to send multipart text+html emails
* Simplify all email templates and convert to Markdown+ERB

This required stripping the CSS and images from the emails. All
Markdown templates are based on the original HTML version except
for the inviter, where it was much easier to work with the text
version (and the i18n contains the whole message anyway).

and improving admin functionality

refactor out profile url generation into local_or_remote_person_path and use it everywhere. closes #2471

RFC2822 specifies that you use parentheses to denote a mailbox list. Use the addr-name spec to specify a display name and the from address.
Check against the raw headers of the mail as @mail.from excludes the display name
Extract shared logic between notifier methods

refactor notifier

This reverts commit a6f2f82c.

fixed emails to show avatars, have subject lines that allow threading in email clients, and relevant content previews

Huge commit I know, but move show and destroy routes into PostsController and PostsController#show into PublicsController

Added confirmation email + specs + User#mail_confirm_email + call User#mail_confirm_email in UsersController#update

do not escape html in private_message text only email