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.
The yes/no pattern didn’t work too well, because:
- it didn’t read naturally as a question and answer
- often users left them completely unclicked if they didn’t want to set
the permission (rather than clicking no)
This commit changes both the invite and edit user pages to use
checkboxes to set permissions. If also rewords these pages to read more
naturally, and explain what the permissions mean.
This meant changing some of the view logic around invites and
persmissions, and I ended up refactoring a bunch of it because I found
it hard to understand what was going on.
