[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.

[user management] users and groups management now really lead to that (closes #1931478) ie, one tab for users, another for groups. By default the groups tab simply list existing groups (with the number of users they count), and propose to create a new group.

[css] use h* title properties from uiprops in cubicweb.old.css (closes #1931476) + minor cleanup of cubicweb.css

[entity fetchattrs] also fetch ambiguous rtypes even if we do not recurse on them (closes #1720823)

backport stable