[#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.
[#44] Fix bug where guide step content wasn't analysed.
[#44] Only see guides when choosing a guide on create new guide step.

Before you could choose any article as the 'guide' for the guide-step.
Now you can only choose articles which are guides.
[#44] Guide steps are indexed and analysed from the guide directly.
[#44] Reduce size of index documents for articles.

1. Call uniq on fields which are likely to contain dupes
2. No need to index keywords since they are derived from the content
[#44] Move text analysis callbacks into RailsNlp included module.
[#44] Move search methods from Article to RailsNlp::QueryExpansion
[#44] Analyse guides when guide steps are created/updated/destroyed.
[#44] Move specs from article to RailsNlp. Tests pass.

Also some slight refactoring to make the stop words method testable in
isolation.