diffexp performance & UX improvements (#431)

* new diffexp REST API spec

* new diffexp REST API; faster diffexp and dataframe slicing

* first draft of fast diffexp

* convert variance calculation to two-pass method

* lint

* update front-end use of API

* fix typo in spec

* disable content compression

* catch index filter format errors

* clean up of dead code

* resolve PR review comments

client/src/util/actionHelpers.js

@@ -31,8 +31,7 @@ export const doJsonRequest = async url => {
   const res = await fetch(url, {
     method: "get",
     headers: new Headers({
-      "Content-Type": "application/json",
-      "Accept-Encoding": "gzip, deflate, br"
+      "Content-Type": "application/json"
     })
   });
   if (res.ok && res.headers.get("Content-Type") === "application/json") {