Invalid links to external URIs in non-mapping properties

[osma]

At which URL did you encounter the problem?

https://finto.fi/yse/en/page/Y382431

What steps will reproduce the problem?

1. Go to above URL
2. Click on the GitHub URL for the home page field

What is the expected output? What do you see instead?

Expected to get to the GitHub URL shown as the link text. Instead getting a page for an empty concept with the URL https://finto.fi/yse/en/page/?clang=fi&uri=https%3A%2F%2Fgithub.com%2FFinto-ehdotus%2FYSE%2Fissues%2F3173

This probably broke in the refactoring that was part of PR #921, in particular the use of ConceptPropertyValue instead of ConceptPropertyMappingValue for non-mapping properties (such as foaf:homepage in the above example)