Handle overflowing tag contents

[allanlasser]

Feedback from @morisy:

Documents that have long metadata strings are cut off and not viewable: https://next.www.documentcloud.org/documents/25057145-oss-correspondence-regarding-senator-byrds-views-on-personnel-promotions/

In KV.svelte, we need to handle situations where the value is a very long string, like a URL.