Remove DiffProposal objects, use Proposal instead

[robinharms]

Remove Diff and use regular proposal instead. No reason to have several objects, add attributes instead.

WAS:
Reactions for DiffProposal objects point to the wrong object

We're pointing them to the base proposal object, but they should point to the main diffproposal object with the same PK.

This causes reactions to deleted diffproposals to remain in the database.

• Create migration
• Update export / import scripts
• Create rest-interface for reactions instead of using message queues