Include service name in page <title>

In pages specific to a service (e.g. dashboard and sub pages) the title
needs to distinguish which service it applies to. This is mainly to give
context to screen reader users who could be managing multiple services.

Implementing this uses template inheritance:

`page_title` includes `per_page_title` includes `service_page_title`

‘GOV.UK Notify’ is inserted into every page title.

Pages that set `service_page_title` get the service name inserted too.

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

@@ -4,8 +4,8 @@
 {% from "components/radios.html" import radios %}
 {% from "components/banner.html" import banner_wrapper %}
 
-{% block page_title %}
-  Create an API key – GOV.UK Notify
+{% block service_page_title %}
+  Create an API key
 {% endblock %}
 
 {% block maincolumn_content %}

app/templates/views/api/keys/revoke.html

@@ -2,8 +2,8 @@
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/api-key.html" import api_key %}
 
-{% block page_title %}
-  Revoke API key – GOV.UK Notify
+{% block service_page_title %}
+  Revoke API key
 {% endblock %}
 
 {% block maincolumn_content %}

app/templates/views/api/keys/show.html

@@ -2,8 +2,8 @@
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/api-key.html" import api_key %}
 
-{% block page_title %}
-  New API key – GOV.UK Notify
+{% block service_page_title %}
+  New API key
 {% endblock %}
 
 {% block maincolumn_content %}