Disable nested inboxes.
Merge pull request #710 from 4teamwork/deif-inbox-nesting

Nested inboxes should not be possible any more
Add proposal type definition.
Make proposal addable to dossiers.
Move model creation to base package.

We now have multiple suv-packages with model definitions, but creating tables
is easier when done in one place only.
Add upgrade step to install meeting profile.
Add basic model implementation with working CRU (sic!) and textindex.
Add tests for proposal creation/querying.
Add proposal one-state workflow.
Add proposal overview page.
Add feature toogle for opengever meeting.
Add proposal listing on dossiers.
Add simple commission/member model.
Add own testing layer for enabled meeting feature.
Move sql-session stuff to opengever.core.model
Generate example commissions.
Allow commission selection for proposals.
Add related documents.
Update translations.
Add workflow configuration/implementation.