Rename Markdown files in keeping w/ GitHub standards.
Member Rewrites: Slim, Serializers, &c.

* Add license notices to main source files/asset manifests.
* Change various models in the `Members` module to use the `Markdown`
    custom field serializer, which stores both plain and rendered
    versions of text side-by-side.
* Add `#create` method on `Api::V1::SessionsController`.
* Add ActiveModel Serializers for models in `Members` module.
* Various small fixes for sundry big bugs.
* Expand and update test suite to cover changes. (100% coverage!)