Unify the UI style in sign up, sign in buttons when using omniauth (GRN-35) (#301)

* Changed style to buttons for sign up

* Added assets and generalized code for sign up buttons

* Added new style to sign in modal

* Replaced hex colors with the official ones

* Removed the old social icons that are no longer used

* Centred text in sign up buttons

* Base button rendering on iconset_providers and secured office 365