[FIX] documents: fix documents creation on documents

1. Fix reply with an attachment on message from existing document

How to reproduce:
- Go to the chatter of a document
- Add yourself as a follower
- Send a message from the chatter: you should receive an email
- Reply to this email with an attachment

You receive a delivery error message ("Address not found") and your reply doesn't appear in the chatter, nor your attachment. 
With this fix, the message with its attachment appears in the chatter and there is no delivery error message.

2. Fix sending a message with an attachment on a request

How to reproduce:
- Go to the chatter of a request
- Send a message with an attachment from its chatter

You get the error "The operation cannot be completed: This attachment is already a document." while the message with its attachment should be added in the chatter.
With this fix, the message with its attachment appears in the chatter and the request remains to be fulfilled.

3. Error while uploading a requested document through activity popover

How to reproduce:
- As user A, create a requested document for User B
- As user B, click on the activity of that request and upload a document

You get the error "The operation cannot be completed: This attachment is already a document." while the uploaded file should be linked to the request.
With this fix, the request is fulfilled, and you get no error.

We solve those problems by not creating document by default when messages with attachments are posted on a document and delegate the creation of the documents to the _message_post_after_hook method for the case of a post on a folder alias. We have chosen that default behavior as in all uses case except the alias, we expect the attachment to be added to the chatter and not as a new document.

TestMailGateway already tests that incoming mail with attachment on an alias create the related documents, and we introduce a test for each case described above in other commits.

[IMP] documents: add a test on replying with an attachment on a document post

We add a test that check that when answering a mail posted on a document with an attachment, that the attachment is added to the document and the message is posted on the document.

We also modify slightly an existing test because a parameter given to _message_post_after_hook was incorrect.

[IMP] documents: add a test on document type change when sending a message

We add a test that check that when sending a message on a URL or a request document, it is not turned into a binary document.

[IMP] documents: add a test that upload a document in the activity popover

We add a test that upload requested document through activity popover.

Task-3707821

Forward-Port-Of: odoo/enterprise#56779