Make page titles consistent

This commit modifies the HTML `<title>` tags for all the pages. It makes two
main changes:
- make the title tag match the `<h1>` of the page, for better or worse
- put the service name after the page title, seperated by an en dash, as per
  GOV.UK

app/templates/views/api-keys/create.html

@@ -3,7 +3,7 @@
 {% from "components/textbox.html" import textbox %}
 
 {% block page_title %}
-  GOV.UK Notify | API keys and documentation
+  Add a new API key – GOV.UK Notify
 {% endblock %}
 
 {% block maincolumn_content %}