[#44] Remove usage of content_md
[#44] Remove documentation about different content fields.
[#44] Remove legacy articles section on admin dashboard.
[#44] Remove support for legacy html articles in article controllers.
[#44] Remove legacy article warning when editing articles.
[#44] Fix db/seeds.rb
[#44] Remove content field from quick answers, web services, guides.

But not guide steps, which use content for the markdown.
[#44] Migration to remove render_markdown, content_md, content.

In the same branch since it can be run after deploying the code.
[#44] Fix search indexing for guides.
[#44] Run migrations (update schema)
[#44] Fix styling bug in seeds file for the published guide.

Step 4:

* Callout should use ***, was using ~~~
* Missing newline in first line of address
[#44] Fix article spec: no longer test for Article#content.