Implement "remove" operation for events

[rikukissa]

Description

The only use case for this operation is when you start a new declaration and click on "Exit" – if this is not done, then the a draft gets stored both in the frontend and in the backend which clutters the app. Need to be deleted from both.

On the backend side, add a constraint that the event can only have a "CREATE" and "DRAFT" actions and no other actions. Otherwise trying to delete it throws an error.

Using this new operation

1. Exit button should delete the draft and go to home view
2. There should be the ... menu and Delete declaration inside it in the Declare action
   

This basically makes the same action but with a different modal. Modals are identical to Farajaland Legacy event views

For attachments:
Implement also a file deletion from the documents service. For this you need to implement a delete endpoint to the service. Users should only be able to delete files they have uploaded themselves. For this reason, you need to amend the POST /files handler in the same service to write the creating user's user id to file metadata.