Track which account is interested in which events

Store a list of ids of events and slots that accounts have marked as
being interested in, and show aggeregate counts in the schedule.

shared/types/schedule.d.ts

@@ -4,6 +4,7 @@ export interface ScheduleEvent {
 	host?: string,
 	cancelled?: boolean,
 	description?: string,
+	interested?: number,
 	slots: TimeSlot[],
 }
 
@@ -18,6 +19,7 @@ export interface TimeSlot {
 	start: string,
 	end: string,
 	locations: string[],
+	interested?: number,
 }
 
 export interface Schedule {