Add db relationship for mainstream categories on organisations

This basically duplicates the relationship between an org and its topics, but with mainstream categories instead.
Add mainstream category selection to org form

Same UI as adding the topics (e.g. a bad UI that just has a fixed number of dropdowns).  Note we've added :inverse_of declarations to the models to allow the validations to pass on create (when creating both the org and the relationship without inverse_of the org on the relationship doesn't exist according to rails).
Show an orgs mainstream categories on their home page

Without any actual designs I've just gone for re-using the design for topics, and placing these categories directly under the topics.