Back office page titles and h1 headings

[joemull]

User story

As a user of the Janeway back office, I would like a consistent name for the pages that I'm visiting. Right now, not all the information is available in the places I'd like, in the structure I need for it to be accessible.

• I can see or hear the name of the page in the browser tab, which is good
  

• I can't see or hear the name of the site in the browser tab

• I can navigate to an h1 with the page title
  

• I am confused by another h1 with the site name
  

Proposed solution

• Agree on a standard format for the browser tab (HTML title) that includes both pieces of info, like Page name | Site name.
• Check extensions on the base template for usage of the existing title block.
• Implement a contextual title, accounting for press, journal, and repository sites, via a standard Django template element on the base template (see #4491 for an example of how we might do it).
• Refactor the HTML around the h1 used for the site name, in the header. Don't use an h1 for this, since there should be exactly one h1 per page.
• Check usage of title-section to make sure we are not creating pages with an empty h1.