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()
 }

toolkit/templates/phase-banner.html

@@ -1,5 +1,5 @@
 {% set phase = "alpha" if alpha else "beta" %}
-<div class="phase-banner phase-banner-{{ phase }}">
+<div class="phase-banner">
   <p>
     <strong class="phase-tag">{{ phase|upper }}</strong> This is a{% if phase[0] in "aeiouy" %}n{% endif %} {% if phase == "beta" %}<a href="https://www.gov.uk/help/beta">{{ phase }} service</a>{% else %}{{ phase }} service{% endif %}  – please send your feedback to  <a href="mailto:enquiries@digitalmarketplace.service.gov.uk?subject=Digital%20Marketplace%20feedback" title="Please send feedback to enquiries@digitalmarketplace.service.gov.uk">enquiries@digitalmarketplace.service.gov.uk</a>
   </p>