Disambiguate names with role and organisation

When selecting a name from a drop down list, editors need to be sure they are picking the correct person. This change adds a person's organisation and role (if they are available) in the drop down text.

https://www.agileplannerapp.com/boards/173808/cards/5172
>  this needs to be Person.includes(:current_roles).all to save a bunch of n+1s. 

:+1:. Also might be best to move it into the helper itself instead of passing it in: 

`options_for_select(disambiguated_people_names, form.object.person_id)`.

Maybe also consider renaming the method to follow the pattern I'm [exploring for the form field caching](https://github.com/alphagov/whitehall/pull/1677/files), e.g. `taggable_people_container`. The use of the word "container" comes straight out the Rails documentation itself around the thing you pass in to the options helpers.