This fixes#3374. There's no good benefit of having possibility to prevent
setting disabled user from beeing an owner.
It can happen that we start preperating a system and use disabled accounts.
Also users shouldn't be forbidden to edit settings on an disabled owner.
- We use the prefix 'py:' for legacy plugins
- Users are now able to insert plugin ID's like 'py:rhodecode.mylegacyplugins.plugin_one'
- Validator tries does an import and checks if the plugin has a 'includeme' attribute.
