Update phase-banner styles

The govuk_frontend_toolkit has outlawed passing in the 'phase' to
the phase-banner components. Colour is now always the same, and it
is now just the text inside the phase tag that changes.

This represents a breaking change for our frontend apps.

Make alpha, beta and discovery colours $govuk-blue, in govuk_frontend_toolkit:
alphagov/govuk_frontend_toolkit#370

toolkit/scss/_phase-banner.scss

@@ -3,12 +3,9 @@
 
 .phase-banner {
   @extend %site-width-container;
+  @include phase-banner()
 }
 
-.phase-banner-beta {
-  @include phase-banner(beta);
-}
-
-.phase-banner-alpha {
-  @include phase-banner(alpha);
+.phase-tag {
+  @include phase-tag()
 }