Add user model

[natsteinmetz]

Fields:

• email (email field)
• name (string)
• experience_lvl (enum)
• position (string)
• active (boolean)
• github_username (string)
• github_id (int)
• github_url (URL field)
• avatar_url (URL field)
• gravatar_url (URL field)

Related to: https://github.com/shescoding/projects-platform-backend/projects/1#card-30647624