imp: Allow to detach documents when posting a message

[marien-probesys]

Related issue(s)

#149

Changes

• add an endpoint to delete a MessageDocument
• return the url to delete a MessageDocument when uploading a file
• when uploading a file, add a "delete" button to detach it
• add a handler to remove a document asynchronously, remove it from the TinyMCE editor and from the hidden inputs

How to test manually

• start a new message
• upload an image
• detach the file → check it disappears from the editor
• post the message → check there is no attachments

Checklist

• code is manually tested
• permissions / authorizations are verified
• interface works on both mobiles and big screens
• accessibility has been tested
• tests are up-to-date
• locales are synchronized
• copyright notices are up-to-date
• documentation is up-to-date (including migration notes)