Undo selection appends genes from differential expression to user gene list (#1183)

* Undo selection appends diffExp genes to user gene list

Fixes https://github.com/chanzuckerberg/cellxgene/issues/1171

Need: When a user performs a differential expression from within
a sub-selection (world) of the data and then resets the selection to all
cells (universe), the differential expression results are no longer
valid.

Approach:
* When the selection is reset, move the top (maxUserDefinedGenes
- len(userDefinedGenes) from the differential expression results to the
list of user defined genes
* Raise maxUserDefinedGenes to 25 to give users more room and
accommodate the extra genes transferred in from differential expression

Other commits:
* Choose different button icons
* Add diff exp genes to user defined genes on subset too
* Respond to feedback from @liaprins-czi and @bkmartinjr

client/__tests__/util/stateManager/controlsHelpers.test.js

@@ -0,0 +1,24 @@
+/*
+test controls helpers
+*/
+import { subsetAndResetGeneLists } from "../../../src/util/stateManager/controlsHelpers";
+import * as globals from "../../../src/globals";
+
+describe("controls helpers", () => {
+
+  test("subsetAndResetGeneLists", () => {
+    const geneList = [...Array(150).keys()].map(() =>
+      Math.random().toString(36).substring(2, 6) // random string of 4 characters
+    );
+    const state = {
+      userDefinedGenes: geneList.slice(0, 20),
+      diffexpGenes: geneList.slice(20),
+    };
+    const [newUserDefinedGenes, newDiffExpGenes] = subsetAndResetGeneLists(state);
+    expect(globals.maxUserDefinedGenes).toBeLessThan(globals.maxGenes);
+    expect(geneList.length).toBeGreaterThan(globals.maxGenes);
+    expect(newUserDefinedGenes).toHaveLength(globals.maxGenes);
+    expect(newUserDefinedGenes).toStrictEqual(geneList.slice(0, globals.maxGenes));
+    expect(newDiffExpGenes).toStrictEqual([]);
+  });
+});