Ability for handler to redact attachments.

[opqdonut]

Sometimes the wrong attachment gets added (e.g. #2488) and needs to be redacted. We could add an API (and perhaps a UI) that would let the owner redact (i.e. zero the contents of) attachments. Redaction could also add an event to the application events, e.g. "Handler redacted attachment X".

• implement an API to redact an attachment (like the other commands in api/applications.clj)
  • clear the attachment contents in DB, possibly in process manager
  • add new event of attachment-redacted with optional comment
• implement action in UI for redacting an attachment similar to review action with checkboxes to filenames
• show event and optional comment for handlers always, and to applicants if redacting an application attachment
  • If the event where the attachment came from is shown to the applicant, then the redaction should be shown too. Otherwise not.
• Show related events could show the event from where the redacted attachment came from.
• Attachment names should be unique (seem to be already e.g. foo (1).png, foo (2).png)

First PR could be the API with the UI later.

[Macroz]

Could be that it should be the handler, in case the handler has added the document.