admin: Rename "deleted" term in associations

Use instead the "excluded" term, which is more broad.
admin: Add setting for showing only active members

Add ADMIN_SHOW_ONLY_ACTIVE_PROJECT_MEMBERS setting, which will limit the
associated users in the details page of a project by showing only the
ones that are accepted members of that project.
admin: Allow showing only active project members

Fix apyrgio/synnefo#243
Merge branch 'feature-admin-project-members' into feature-admin