Lack of upload permissions for guests breaks some apps

[ssergio-ll]

When I register guests, they have access to Nextcloud, but there are certain things they cannot do, resulting in errors. Among the issues I've observed are:

• They cannot upload files to cards in Deck.
• They cannot send files in Talk.
• They cannot send audio messages in Talk.

All of those actions end in an error.

[small1]

A guest user cant upload files to their home directory. And as talk and deck save stuff in folders in the usres home directory they are getting the error. This is pretty much how it works.

It would be good to have apps like talk and deck be able to override stuff. (There are other situations as well where we get a few probems)

[joshtrichards]

Related: #1091

[solracsf]

Please note that Constants::PERMISSION_CREATE alone would not solve this; for Deck, as an example, Constants::PERMISSION_SHARE would also be needed as technically, a share is made after the upload.