MDL-27814 blogs: Cleaning up contexts in blogs

All blogs pages should be in system context. Blogs are in system context just about various things (course, activity etc)
MDL-27814 blogs: Remove redundant code

Since all blog pages are in system context always, it is useless to try to get course/mod params from page context
MDL-27814 blogs: Deprecate blog_get_context_url()
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.