Admin provided file templates

Currently file templates are only available as a special folder in the user directory. This also means that we can only update templates on first login of the user or when the user removes and recreated that directory.

With Collabora we already have a admin section to configure instance-wide templates, which from my perspective would be useful for all file types as a general feature in server. This would also allow us to update them if we ship new templates and also allows admins or organisations to maintain shared templates and provide updates to those that are becoming available to all users automatically.

We could then leave the user directory as something to be explicitly managed by the users.

Also allows us to unify the implementation and not have something special in the collabora app.

cc @jancborchardt

Start by reviewing how templates are currently supplied through the special user-directory folder and how the Collabora admin section configures instance-wide templates. Map the requirements for a shared server-level template store, including administrator updates and support for all file types. Done means users can access centrally managed templates while their personal template directory remains explicitly user-managed.