FIX: Failed incoming emails could create empty topics

A failure condition is eliminated where a topic would be created, but post
creation would fail, leaving the forum with a topic without any posts.
By asking PostCreator to create the topic instead, inside of its
transaction, this failure condition is eliminated.

Additionally, attachments are restored to working status. Previously,
the attachment code would build up the post raw, but then drop it and
not do anything with the result (creating orphaned uploads). By actually
placing the raw value back in the options hash, it is included in the
created post.
Add Email::HtmlCleaner for email processing

This class is in charge of stripping out most of the crap from the HTML
portion of emails that email clients generate, so that it can be sanely
post-processed for signatures and quoting boundaries.
FIX: Prefer HTML in incoming emails, heavily refactor email receiver

This commit heavily refactors Email::Receiver to both better handle
different emails and improve testability.

A primary focus of the refactor is reducing the usage of class
variables, in favor of actually passing parameters - making it possible
for multiple tests to use the same Receiver instance.

The EmailLog reported when a topic is created is reflected to put the
user's email in the to_address field, instead of the system address.

The discourse_email_parser function is renamed to
discourse_email_trimmer, and additional stopping conditions are added to
make up for EmailReplyParser's inability to deal with html at the start
of a line.

The force_encoding calls are refactored out to a 'fix_charset' method.

parse_body is renamed to select_body, and the scrub_html method is
dropped in favor of the new HtmlCleaner class.

A new parse_body method is added, which performs the job of the removed
lines of code in the 'process' method.

EmailUnparsableError is redefined again, to be encoding errors (when the
declared encoding is not what was delivered).
Update the Receiver and PollMailbox specs for the changes

Tests are both added, moved, and deleted.

Add test for topic not being created

Move html_only.eml to parse_body testing section
Have parse_body() recover from ASCII-8BIT encoding

Added a test to make sure that the result can be passed into TextCleaner
(which expects UTF-8)
Merge pull request #2703 from riking/email-pr-receiver

Heavy refactor for Email::Receiver