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/templates/views/edit-sms-template.html

@@ -18,7 +18,14 @@
           {{ textbox(form.name, width='1-1', hint='Your recipients won’t see this') }}
         </div>
         <div class="column-two-thirds">
-          {{ textbox(form.template_content, highlight_tags=True, width='1-1', rows=5) }}
+          {{ textbox(
+            form.template_content,
+            highlight_tags=True,
+            character_count=True,
+            width='1-1',
+            service_name=current_service.name,
+            rows=5
+          ) }}
           {{ page_footer(
             'Save',
             delete_link=url_for('.delete_service_template', service_id=current_service.id, template_id=template_id) if template_id or None,