[JRO] Rm attachments and translate `t:img` tags to image tags

The current way where we add attachments to a post via the nested attributes -- posts having many attachments, etc -- is a poor method of adding images to posts. As a result the app should allow external methods of doing this. In my mind this should be another engine handling file uploads and displaying it.

So - this commit is the first step to removing these. Instead of completely removing attachments for now we'll leave them in there and will, instead, translate all `[t:img]` tags to real image tags (in markdown or bbcode)

* Remove `accepts_nested_attributes_for :attachments` from Post.
* Add a rake task -- `thredded:update_legacy_timg_tags` -- to make the translation in each post.
* Add two traits for post - `:markdown` and `:bbcode`
* Remove the attached image filter
* Remove attached files form fields
* Remove ability to attach images via email
[JRO] BREAKING NEWS: I am an idiot.

Until now there were people not getting @ mention notifications because they had not updated their profile / settings. This commit will create a default and will send them their @ notification if so.

* Swap double negative for something more human
* use `first_or_create` instead of the `try` to ensure a record exists - either a default record or a previously set one.
* Derp.