Social media absolute url config

[manics]

What this PR does

Allows absolute URL for social media to be overriden by the OMERO.web administrator, the default is to use Django's build_absolute_uri which probably uses the Host header sent by the client.

--depends-on #5244

In conjunction with #5244 this includes all required changes from #4744 (some commits were already rebased to develop, others are unnecessary following the addition of omero.web.api.absolute_url in mainline.

Testing this PR

1. Configure a public web user, enable social media options, e.g. https://github.com/manics/deployment/blob/9bba3a5c7223acb6b9f01c16fc5435778796d2e8/ansible/group_vars/omero-hosts.yml#L109
2. Check the twitter and opengraph metadata is present in the HTML source of the full image viewer
3. Set a custom value for omero.web.api.absolute_url, check this is reflected in the hard-coded URL.

Related reading

https://trello.com/c/z2U5K6ht/1-omero-5-3-migration

@aleksandra-tarkowska

[manics]

--rebased-from #4744

[atarkowska]

works as expected with #5244

<meta name="twitter:image" content="https://test.example.org.custom.domain/prefix/webgateway/render_thumbnail/1229801/280/">