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cells (universe), the differential expression results are no longer
valid.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/src/components/menubar/subset.js

@@ -0,0 +1,48 @@
+import React from "react";
+import {AnchorButton, ButtonGroup, Tooltip} from "@blueprintjs/core";
+import * as globals from "../../globals";
+
+
+function Subset(props) {
+  const {
+    subsetPossible,
+    subsetResetPossible,
+    handleSubset,
+    handleSubsetReset,
+  } = props;
+
+  return (
+    <ButtonGroup style={{marginRight: "10px"}}>
+      <Tooltip
+        content="Subset to currently selected cells and associated metadata"
+        position="bottom"
+        hoverOpenDelay={globals.tooltipHoverOpenDelay}
+      >
+        <AnchorButton
+          data-testid="subset-button"
+          active={!subsetPossible}
+          icon="pie-chart"
+          onClick={() => {
+            if (subsetPossible) handleSubset();
+          }}
+        />
+      </Tooltip>
+      <Tooltip
+        content="Undo subset and show all cells and associated metadata"
+        position="bottom"
+        hoverOpenDelay={globals.tooltipHoverOpenDelay}
+      >
+        <AnchorButton
+          data-testid="reset-subset-button"
+          active={!subsetResetPossible}
+          icon="full-circle"
+          onClick={() => {
+            if (subsetResetPossible) handleSubsetReset();
+          }}
+        />
+      </Tooltip>
+    </ButtonGroup>
+  );
+}
+
+export default Subset;