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`.
Return the proposal identified by the given oguid or None if no such proposal exists.