MDL-1626 mod_forum: Add a function to determine if the user is subscribed to a discussion
MDL-1626 mod_forum: Per-discussion subscription toggling
MDL-1626 mod_forum: Add unit tests for forum subscription changes
MDL-1626 mod_forum: Update discussion subscriptions when moving discussions
MDL-1626 mod_forum: Move subscription content to new class and rewrite caching