Resolve "Make events moddable"

Closes #763 (closed).

Changes:

• Allows mods to load event phases in onBeforeGetEventPhases (works the same as modded status effects)
• Changes afterGiveRewards to giveRewards (since there's no built-in reward giving, it isn't an after—lmk if I should rename this event) and moves fishing rewards into fishingTournament.js
• Adds afterJoinEvent for events to hook into and moves fish removal (when joining the event) into fishingTournament.js

Edit: Also let me know if I should merge into v0.3.1 or master instead of v0.3.1-rebased since branches are being switched around right now.

Edit 2: Updated this branch with current master, should be merge-able without conflicts.