[test api] rename _user_session to user_groups_session, fix its arguments and return only the session, not (user, session) to make things clearer

auto-delete relation with single cardinality using execute, not unsafe_execute, so permissions are correctly checked

fix tests

add */*.py to default MANIFEST.in to avoid pb such as #719282 in the future

fix #615379 by checking admin user exists instead of making think everything is fine

cleanup, typos

[form] fix #719285, due to multiple calls to restore_previous_post, by proper refactorings * move __init__ code from FieldsForm to Form. Must behaviour here should actually be in the Form base class * avoid buggy duplicated call to restore_previous_post * move some code that was in the form renderer to the form'__init__ method (__redirectpath & __form_id hidden input handling)) * 'formvid' should now be specified on form selection, not on form rendering