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

config/locales/en.yml

@@ -100,6 +100,10 @@ en:
       with: Sign in with %{provider}
   omniauth_error: An error occured while authenticating with omniauth. Please try again or contact an administrator!
   password: Password
+  provider:
+    google: Google
+    microsoft_office365: Office 365
+    twitter: Twitter
   recording:
     email: Email Recording
     no_recordings: This room has no %{inject}recordings.