URIs on enumeration pages do not link out to external vocabularies

[AlasdairGray]

Describe the bug
When an enumeration is declared to be linked to an external vocabulary using the enum_uri property, the link to the external vocabulary is not generated by the LinkML framework.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'https://co-cddo.github.io/ukgov-metadata-exchange-model/FrequencyValues/'
2. Displayed URI is not to the Dublin Core vocabulary but a self link to the page

Expected behaviour
The FrequencyValues enum has the enum_uri: http://purl.org/cld/freq/. I would expect this value to be displayed to the user. This would then match the behaviour of declaring the class_uri for a class, see Dataset.

Additional context
This was discovered while fixing #61. The fix for #61 means that 404 errors are not generated but does not fix this issue.

This has been raised as an issue on the LinkML framework linkml/linkml#1682 (comment)

[AlasdairGray]

Currently tagged with won't fix while we wait for a response from the LinkML team.