Add initial meeting implementation.

This PR adds an initial meeting implementation.
- Add Proposal content type (proxies) and model.
- Add feature flag, disable meeting feature by default.
- Add Committee content type (proxy) and model.
- Add custom forms to create/edit proxy-supported entities.
- Add custom workflow implementation (no plone workflow).
- Add a context manager to temporarily change the security context.

Furthermore in an attempt to improve import dependecies some code is moved to `opengever.base`.
Just a minor thing but I find the following format much easier to read and shorter:

```
        return [
            {'label': _('label_title'),
             'value': model.title},
            
            {'label': _('label_initial_position'),
             'value': model.initial_position},
            
            {'label': _('label_committee'),
             'value': model.committee.title},
            
            {'label': _('label_workflow_state'),
             'value': self.get_state().title},
        ]
```