Gracefully handle tagger/validator params in both validation and tagging mode

[iross]

Right now there's a disconnect with the frontend in terms of what's being provided and used in the validation and tagging modes. The frontend isn't providing a tagger with POST of a new annotation set, and the API is not using any logic to recognize (ignore) the tagger field when tags are being validated (see UW-COSMOS/cosmos-visualizer#69)