Make UUID regex match only exact UUIDs (currently, it'll match strings with UUIDs anywhere in the string)

I implemented the original fix to handle UUIDs sometime last year, and just now realized this issue. The UUID regex needs to match exact UUIDs. Right now, the UUID regex passes when there's a UUID at the end of a slug, which is what occurs for duplicates. This can result in errors like
````
PG::InvalidTextRepresentation: ERROR:  invalid input syntax for uuid
````
when it should instead result in a RecordNotFound error.