feat(webhooks): add webhooks to user.preferences (API settings) so we can post to external services (eg, Beeminder)
Merge branch 'webhooks' into develop

Conflicts:
	src/controllers/user.js
feat(webhooks): move webhooks from array to object (everything should
really be stored as objects). Add to API routes (see
HabitRPG/habitrpg-shared#83a22b8). Note, sorting doesn't work. Meh.