Implement endpoint for committee-specific leaderboard

[nkansal96]

Motivation

It would be useful to know which members attend your committee events the most. This information could be used for prizes, shoutouts, or recruiting interns/officers.

To Do

• [ ] Create a new db function called getCommitteeLeaderboard(committee, offset, limit) that queries the database

• [ ] Extend the leaderboard api to include a /app/api/v1/leaderboard/:committee? endpoint This would respond with the users and the number of events from that committee each user has attended. We won't be doing this by points because we don't store points by committee, so it'll be a little complicated and point information is not as useful to admins.

[HirdayGupta]

Notes to self:

• Similar to membership-portal/app/db/schema/user.js

[HirdayGupta]

Some docs and tutorials that I looked at for joins:

• http://docs.sequelizejs.com/manual/tutorial/associations.html
• https://lorenstewart.me/2016/09/12/sequelize-table-associations-joins/