Track labels from individual users

[sauln]

From black_holes_frontend created by sauln : codeforgoodconf/black_holes_frontend#5

Keep track of the labels that a user gave and the labels that came from which person.

With this information, we can enable other features:

• treat given labels as votes, only confirming the label if there have been enough or unanimous votes.
• rate the accuracy of each user, with options to weight users' votes or reject them from participating.
• retract labels from users.
• enable training from Add Gaussian smoothing to processed_data #4 and track improvement.

TODO:

• Build a user model
• Build a vote model
• Connect spectra->vote with 1-many relationship
• Connect user->vote with 1-many relationship