Refactor merge_otus to request body as NexSON.

We learned during the Save Study feature that it's painfully slow to
encode a full study's NexSON, so instead we should pass it raw in the
request body and teach the server to look for it there. This uses code
adapted from the Open Tree API.
Merge OTUs and refresh the UI after tree import.
Misc fixes for merge_otus and curator error view
Restore curator's dedicated error page
Fix final nexml ref after merging OTUs
Fix missing import (HTTP exceptions)
Merge pull request #251 from OpenTreeOfLife/merge-otus-and-refresh

Merge otus and refresh