BatchSender: using CORS POSTs to send the data

**PLEASE DON'T MERGE**

This gets rid of the img GETs in favor of CORS POSTs. BatchSender is way simpler now, woot!

This still needs to be tested more, TRUST being down this weekend is preventing that for now. But want to get this out there for feedback.
Is there any advantage to encoding this early instead of in the _makePOST function. Practically, it doesn't matter, but it seems like sending is the logical place for encoding to happen.