Move users.prefs to a separate table

[briri]

Move the existing users.prefs column into a separate table. The data should be stored as a JSON string. The site runs queries against the users table for every request. Moving this data to a join table will prevent it from being loaded unless needed.

We should also update the logic so that its only storing deviations from the default preferences.

related to: #146

[xsrust]

• New Prefs table is created
• Users have_one Pref; Prefs belong_to User
• Existing prefs stored under user.pref.settings[:email]
• New types of prefs can be stored in user.pref.settings[:key], e.g export preferences
• Users will not have a pref until they manually save a change from the default. (i.e. ensure you check for nil, or use the helper function get_preferences(key) in user to access them.

[xsrust]

This Work will require the following actions before it can be deployed:

• Rake db:migrate
• Updating branding.yml to follow the pattern of example_branding.yml
  • Defaults need to be set here for all of the preferences

[stephaniesimms]

Closing this as there doesn't seem to be anything to test in the UI