New URL scheme works now

Examples:
plexode.com/vorp#level-01 plays level 1
plexode.com/vorp#level=level-01 also plays level 1
plexode.com/vorp#level-01&mode=edit edits level 1
plexode.com/vorp#level-01&mode=json
Soon builtin levels won't be editable though, so these URLs won't ever
work in production.
Also fixed over-allocation of Audio Contexts from level to level, which
causes errors.