Move events access control into authorization module

In #1961 @amitupreti refactored the access checks into a separate module in the project app, so that all project authorization is done in one place, which has made project access in the code a lot easier to reason about. There are effectively two functions in the authorization module:

• get_accessible_projects(user) - list out all projects a user can access
• can_access_project(user, project) - can the user access the specified project?

(Side note: in my opinion, the reason to have the latter function with separate functionality is it is a hot path, and it might be worth optimization. Otherwise, it is trivially derived from get_accessible_projects(user).filter(project=project)).

Currently the events/models.py app skip the authorization module entirely:

https://github.com/MIT-LCP/physionet-build/blob/851735ff071b7d356e5b1c65061c02cae00be01a/physionet-django/events/models.py#L225-L241

This code should be moved into the authorization module. To keep it simple, creating a new has_event_access(user, project) function to the authorization module makes sense. Removing redundancy between has_event_access and has_access can be worked out in the future, and similarly improving the mechanism through which datasets are provisioned via events can equally be improved.