Custom query support for autocomplete model

This lets you provide a third parameter to 'autocomplete_model' which
can asynchronously return a query object. This will allow us to restrict
the list of auto-completed to include only those that are relevant to
the currently selected post.
Pass selected post's id when autocompleting orgs

This will allow us to filter the list of organization returned to only
include ones that are appropriate for the selected post.
Add custom query for organization auto-complete

Only auto-complete organization which are relevant to the currently
selected post.
Merge branch '418-restrict-posts-listing'