How to Reproduce : 
1. Remove 'Create' access on the 'Knowledge Article' model for Portal and 
   Internal users.
2. Now log in as a Portal.
3. Try to create a new private article.
4. Log in as an Internal user.
5. Try to create a new private article.

Both Internal and Portal users can still create private articles even after
'Create access' is removed.

Article creation logic in `knowledge.article` was bypassing the usual access
rights because of `sudo` (mainly to add the creator as a member, since creation
rights on the member model are not granted). This allowed users to create
private articles even without create access.

This commit introduces:
1. Model-level access check in `create`. Sudo the creation of articles only when
   the user has create rights.

2. UI imp to hide the '+' button in the sidebar and the `New` button in
   the topbar when the user doesn't have create access.

3. New test cases to verify that model-level access rights are respected when creating an article.

task-4916280