Update UNKNOWN terms in UI display and export files

[dustine32]

From https://github.com/geneontology/private-functionome/issues/10#issuecomment-2691594616.

1. Update the labels for UNKNOWN terms to "unassigned" instead of "unknown".
2. Remove unknown annotations from export files
   a. GAF
   b. CSV
   c. JSON

In the CSV, I see lines like:

UniProtKB:Q9Y5V0        ZNF706  Zinc finger protein 706         Unknown cellular component

[dustine32]

@tmushayahama has noted that there are some hard-coding of "UNKNOWN" in the UI code. This will require a bit more work from @tmushayahama.