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.

tests/app/main/views/test_templates.py

@@ -29,10 +29,14 @@ def test_should_show_page_for_one_templates(app_,
                 template_id=template_id))
 
     assert response.status_code == 200
-    assert "Two week reminder" in response.get_data(as_text=True)
-    assert "Your vehicle tax is about to expire" in response.get_data(as_text=True)
+    content = response.get_data(as_text=True)
+    assert "Two week reminder" in content
+    assert "Your vehicle tax is about to expire" in content
+    assert 'data-module="character-count"' in content
+    assert 'data-service-name="Test Service"' in content
     mock_get_service_template.assert_called_with(
-        service_id, template_id)
+        service_id, template_id
+    )
 
 
 def test_should_redirect_when_saving_a_template(app_,