MDL-1626 mod_forum: Add unit tests for forum cron mail behaviour
MDL-1626 mod_forum: Create new discussion subscription tracking table
MDL-1626 mod_forum: Make cron handle discussion subscriptions
MDL-1626 mod_forum: Add functions to change user discussion subscription state
MDL-1626 mod_forum: Change to per-discussion subscription option in new post form
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