FEATURE: Cook drafts excerpt in user activity (#14315)

The previous excerpt was a simple truncated raw message. Starting with
this commit, the raw content of the draft is cooked and an excerpt is
extracted from it. The logic for extracting the excerpt mimics the the
`ExcerptParser` class, but does not implement all functionality, being
a much simpler implementation.

The two draft controllers have been merged into one and the /draft.json
route has been changed to /drafts.json to be consistent with the other
route names.

spec/requests/session_controller_spec.rb

@@ -478,7 +478,7 @@ RSpec.describe SessionController do
       UserAuthToken.destroy_all
 
       # we need a route that will call current user
-      post '/draft.json', params: {}
+      post '/drafts.json', params: {}
       expect(response.headers['Discourse-Logged-Out']).to eq("1")
     end
   end