Tidy up admin topics index view

Fix an N+1 query and drop unnecessary column
Drop count caches from classification

All topics now have policies and documents, and have had so for a
while. The counts are no longer serving any purpose.

Also adds a pending migration to drop the database columns.
Whitespace
Add TopicSuggestion model
Code style
Add topic suggestion field to edition form

Also validate that there's an associated topic or a suggestion.
Test edition topics validation
Update factories for topics validation