Context:
The custom project templates feature uses project export to copy the
template data to the new project.

Problem:
Public project templates, with restricted features access, like issues
restricted to the project members only, were having the protected data
leaked to the new project since the feature access level wasn't being
validated on the `User#available_custom_project_templates`.

Solution:
Validate the user access to all the exportable features before listing
the project as an available custom project template to the user.