Fixes broken login via form (DDR-1616)

There were several issues at play here, not necessarily in order.

• There was a devise initializer in ddr-admin that conflicted with the one in ddr-core. Notice that dul-hydra does not have a devise initializer (ddr-models has one).
• Devise 4.x requires explicit configuration of permitted controller params for various actions (sign_in, sign_up, etc.) beyond defaults that include password.
• The Users::SessionsController in ddr-core was overridden (temporarily) until we can review it.
• The sign_in and sign_up forms had to be customized to include a username field, which is the authentication_key we are using.