Add Endpoint to store mislabeled tweets

[Haimchen]

Goal

Create a new public API endpoint where mislabeled tweets can be sent.

Description

Add a new endpoint POST /mislabeled_tweet to the public API.
It should accept the following parameters:

• text (string): the text of the tweet (html removed?)
• tweet_id (string): ID of the tweet
• prediction_value(float): the prediction value the model returned for the tweet
• user_threshold (float): the current threshold value the user has set
• model_version(int): the model version the prediction was made with

The new feedback entries should be stored in a new database table, feedback.

Tasks

• add new database table
• add new route
• add new endpoint to README

[djbusstop]

Not sure if we should store this data exactly the way described but every thing else is generally correct. #13