amendment events

This introduces events for amendments.

We introduced amendments as a hack for #535. They are basically proposals but have a very different user interface.

Amendments are currently handled as proposals. However, emitting `T_PROPOSAL_CREATE` events is disabled for amendments while `T_PROPOSAL_EDIT` and `T_PROPOSAL_DELETE` are not.

This...

-   creates amendment event types `T_AMENDMENT_CREATE`, `T_AMENDMENT_CREATE` and `T_AMENDMENT_CREATE` as well as a set `S_AMENDMENT`
-   emits amendment events instead of proposal events if the proposal is in fact an amendment
-   provides a new event icon for amendment events
-   provides previews in dashboard and profile page

In combination with #607 this should improve the amendment usability quite a bit.