MDL-1626 mod_forum: Create new discussion subscription tracking table
MDL-1626 mod_forum: Move subscription management to a separate class
MDL-1626 mod_forum: Add functions to change user discussion subscription state
MDL-1626 mod_forum: Make cron handle discussion subscriptions
MDL-1626 mod_forum: Add icons for subscription status
MDL-1626 mod_forum: Update user interface to reflect per-discussion subscriptions
MDL-1626 mod_forum: Update discussion subscriptions when moving discussions