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.
Because users have difficulty getting back to the Notify admin
interface.
The `rel` attribute mitigates against [a nasty cross-domain
vulnerability](https://mathiasbynens.github.io/rel-noopener/).
