Added document support to events

Ghost User
2018-12-19T22:46:18+01:00

Closes #257

### Short description

This MR allows adding documents to events, and adds a new `EventDocument` submodel of `Document` which allows committees to create and own documents.

### Steps to use this feature

1. Create a new committee
2. Create a new event organized by the committee created in the previous step.
3. Create a event document owned by document.
4. Add the newly created document to the new event.
5. Go to the event page, there should now be a new section with a list of documents.