Use symbols instead of strings for Twitter config

Switch back to using symbols instead of strings to load Twitter secrets
and keys.
Create migration to allow null emails

Omniauth authentication providers do not always return a user's email
in the info hash sent back through the callback. Providers like Twitter
keep that information private/hidden. User records need to be able to
be saved without an email if they're using an omniauth provider.
Fix and run email migration

Fix the table name to be changed (should be plural) and run the
migration over the users table.
Test Twitter settings before loading strategy

Before attempting to load Twitter configuration into the devise
strategy, confirm that Twitter is enabled from the custom environment
settings (config/environment_custom.rb).
Change user validations for omniauthed Users

Change user validations:
  * Devise validates user emails so it does not need to be validated
    here.
  * Users cannot have the same provider and uid combination as other
    users. However, users do not need to have an omniauth provider and
    the column can be left blank.
Override Devise's email validation

Devise requires emails by default. However, since the application uses
omniauth providers that don't make user emails addresses available
(Twitter), a user only needs to have one if they do not have an
omniauth provider.
Finish Twitter omniauth

Solution for signing in Omniauthed users should be generic and allow
for new strategies to be added later. After user credentials have been
authenticated with the third party, information is passed back to the
controller and handled generally. User records are created or found
based on the provider/uid combination, which should be unique.
Update User model spec and factories

User model specs were never updated to user the user factory. Adjust
the specs to use it and more exhaustively check the validitiy of user
records.

Add tests for users registered though third parties with omniauth and
the method for finding or creating them.