Switching fkeys on Discussion and Synthesis.
Fixing Discussion.posts() docstring.
Adding method Synthesis.serializable()
Synthesis API Endpoint
Adding method Email.reply()
Adding method `Mailbox.most_common_recipient()`.

It returns the email address of the most common recipient in its contents.
Adding method `Mailbox.email_most_common_recipient()`.

It's for sending an email to the most common recipient in the mailbox.
Adding method Mailbox.serializable()
Post to /posts/ endpoint.
Merge branch 'develop' of https://github.com/ImaginationForPeople/assembl into develop
Ignoring root_post_id in get_posts when == 0.