MDL-27814 blogs: Deprecate associate blog caps

moodle/blog:associatemodule and moodle/blog:associatecourse should not be used
anymore as per the discussion in the issue. Everyone is free to blog about anything
they want to blog about.
MDL-27814 blogs: Refactoring code to use proper context and such

Based on the decisions made in the issue, most places in blog should use site context.
There are also minor other refactoring to support all changes made in the issue.