Add a phase-banner class and update examples

Keep the .phase-banner-alpha and .phase-banner-beta classes, so this
won’t be a breaking change, but since there isn’t a difference between
the two (as we’re not passing in a state to change the colour of the
phase tag), use a single class - phase-banner for both examples.

app/views/includes/phase_banner.html

@@ -1,4 +1,4 @@
-<div class="phase-banner-alpha">
+<div class="phase-banner">
   <p>
     <strong class="phase-tag">ALPHA</strong>
     <span>This is a new service – your <a href="https://designpatterns.hackpad.com/GOV.UK-elements-feedback-sKrDyQxcfA2">feedback</a> will help us to improve it.</span>

app/views/snippets/phase_banner_alpha.html

@@ -1,4 +1,4 @@
-<div class="phase-banner-alpha">
+<div class="phase-banner">
   <p>
     <strong class="phase-tag">ALPHA</strong>
     <span>This is a new service – your <a href="#">feedback</a> will help us to improve it.</span>

public/sass/elements/_phase-banner.scss

@@ -1,10 +1,8 @@
 // Phase banners
 // ==========================================================================
 
-.phase-banner-alpha {
-  @include phase-banner();
-}
-
+.phase-banner,
+.phase-banner-alpha,
 .phase-banner-beta {
   @include phase-banner();
 }