UX: Add THEME_SITENAME as a suffix in <head>

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Description

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Currently the <head> is either the "<sitename>" or some custom string like "Communities", "Requests" etc. This PR results instead in "Communities | <sitename>", "Requests | <sitename>", etc

[ ] Should I also change this in the other theme https://github.com/inveniosoftware/invenio-theme/blob/master/invenio_theme/templates/invenio_theme/page.html#L27 ?

Checklist

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[x] I'm aware of the code of conduct.
[x] I've created logical separate commits and followed the commit message format.
[ ] I've added relevant test cases.
[ ] I've added relevant documentation.
[ ] I've marked translation strings (for relevant code).
[ ] I've followed the CSS/JS and React guidelines (for relevant code).
[ ] I've followed the web accessibility guidelines (for relevant code).
[x] I've followed the user interface guidelines (for relevant code).
[ ] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
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Close https://github.com/zenodo/ops/issues/390

inveniosoftware / invenio-theme

UX: Add THEME_SITENAME as a suffix in <head> #382

Description

Checklist