[refactoring] introduce add_etype_button function to properly create button to add a new entity also properly use cw_create_url to generate the URL to add the entity

[user prefs] use same title as the action for consistency

[sources management] properly set __linkto, closes #1922970 when clicking on 'add mapping part' on an external source view, one expect to have source properly selected in form

some minor cleanups

[web test] fix test broken by new users/groups management views

[table view] allow to set None in 'headers', meaning the label should be fetched from the result set as done by default (closes #1931533) the point of this is to avoid having to define the whole headers list when only some of them have to be defined.