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.
Heavily refactor Email::Receiver

A primary focus of the refactor is reducing the usage of class
variables, in favor of actually passing parameters. Additionally, 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 the
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.

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 yet again redefined to be encoding errors.
Update the Receiver and PollMailbox specs for the changes

Tests are both added, moved, and deleted.