Filter out seen subjects from the subject queue (#4571)
Filter out seen subjects from the subject queue (#4571)

* Clean up some linter errors
Use named class methods instead of anonymous closures.
Remove an unused import.
Remove some semicolons and commas.

* Filter subjects queue for seenThisSession.

amyrebecca/Panoptes-Front-End,zooniverse/Panoptes-Front-End,amyrebecca/Panoptes-Front-End,amyrebecca/Panoptes-Front-End