Add attachment with one API call

[opqdonut]

Currently, an attachment is added and then a draft-saved command is issued to store the reference to the attachment in the application

Instead, we should have an API endpoint like /application/add-attachment?application-id=123&field-id=7 that would add the attachment to the db and emit a draft-saved event. This would simplify the logic on the UI side.

[Macroz]

Adding and removing of the attachment should be the same. There should also be no need to press save button separately.