Make config items fully editable

fixes #694
Remove check on undefined module and key

To prevent module and key to be saved with values “undefined”, the
values are normalized to empty strings in the form before getting
passed.
Don't create new entry on change.

Bring back update function to delete entry when module or key name have
not changed.
Merge branch 'ac/admin_edit_config'

Fixes #700.
core: Set z_pid cookie only when set_persistent is used. Fixes #701
Merge branch 'master' of https://github.com/zotonic/zotonic

Conflicts:
	modules/mod_admin_config/templates/admin_config.tpl
	src/support/z_session_manager.erl