allow users to change user_name (if available)
consolidated all message-related migrations into one

Except for mailgun_data, that is. But I want to keep that separate as it's
pretty hacky and the plan is to get rid of it eventually.
consolidated identity-related migrations
consolidated user-related migrations
only initialize Pusher if the environment is set up properly
added Message#archive!
fixed a spec to be order-agnostic
made a few tweaks to improve spec output readability