Add character count as you type

Text messages have a maximum length, which we tell the users. We
shouldn’t expect people to count the characters in the message
themselves.

This commit borrows [the word counter from the Digital Marketplace
frontend toolkit](9d17690de5/toolkit/javascripts/word-counter.js)
and adapts it to count characters instead.

Things I’m still not sure about with this:

- what should it say when the message goes over the length of one text
  message
- what’s the interaction with placeholders, which will change the length
  of the message

This commit also adds a line to the pricing page which explains that
service name counts towards the length of the message.

app/assets/stylesheets/components/textbox.scss

@@ -54,3 +54,9 @@
 .textbox-right-aligned {
   text-align: right;
 }
+
+.textbox-character-count {
+  @include core-19($tabular-numbers: true);
+  margin: 10px 0 0 0;
+  color: $secondary-text-colour;
+}