MDL-1626 mod_forum: Add functions to change user discussion subscription state
MDL-1626 mod_forum: Basic tests for per-discussion subscription
MDL-1626 mod_forum: Change to per-discussion subscription option in new post form
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