Add description to image search fields

[calvinmclean]

Description

Simply add the 'description' field of the Application/Image model to the search_fields attribute of the ImageViewSet.

Checklist before merging Pull Requests

• New test(s) included to reproduce the bug/verify the feature
• Add an entry in the changelog
• Documentation created/updated (include links)
• If creating/modifying DB models which will contain secrets or sensitive information, PR to clank updating sanitation queries in roles/sanitary-sql-access/templates/sanitize-dump.sh.j2
• Reviewed and approved by at least one other contributor.
• New variables supported in Clank

[coveralls]

Coverage decreased (-0.01%) to 39.275% when pulling 116c8eb on calvinmclean:image_search into 2e299d4 on cyverse:master.