Allow users to participate in multiple events at once

[alexrandaccio]

Is your feature request related to a problem? Please describe.
Formerly, the Delegate model extended the default User class provided by Django via a OneToOneField. Each user's voice credit balance was recorded in the credit_balance field of their Delegate object.

However, we want users to be able to participate in more than one event on the app. This means they have to have a separate voice credit budget for each event. In order to implement this, we need a new database model.

Describe the solution you'd like
New structure: each user has one Profile object that extends the User class. For each event that a user is invited to, a new Delegate object is created to record their voice credit balance for that event. See below:

User (holds default Django login info)

• email
• password
  etc.

Profile (holds other account info necessary for our app)

• user (OneToOneField)
• public_username
• is_verified
• oauth_token
• groups_managed (for group admins)
  etc.

Delegate (holds account info related to one specific event the user is participating in)

• profile (ForeignKeyField)
• event (ForeignKeyField)
• credit_balance
  etc.