Define moderator roles for users

[angelocordon]

Feature Story:
As a moderator, I should have the ability to add users as moderators so that we can have more help moderating the platform.

[angelocordon]

cc: @lpatmo

I'm foreseeing that this issue might be blocked based on how the Moderator Dashboard pans out (#15). I was originally thinking that we could SSH into our servers and add moderators through the console, but from what I can tell, Meteor doesn't seem to be available in our staging server (calling meteor in the staging terminal returns Meteor not found).

We could connect to our Mongo db using the Mongo shell locally to do it manually as needed, but we'd still need to the modified user schema in place to do that.

[lpatmo]

We need three levels:

• admins
• moderators
• users

Only admins can promote users to be moderators.

For the MVP, we can get away with just changing specific users to be moderators (moderator: true) via the DB.