webui: create label bullets in valid labels filtering

webui/src/components/LabelBullet.tsx

+import React from 'react';
+
+import { common } from '@material-ui/core/colors';
+import { makeStyles } from '@material-ui/core/styles';
+import {
+  getContrastRatio,
+  darken,
+} from '@material-ui/core/styles/colorManipulator';
+
+import { Color } from 'src/gqlTypes';
+
+import { LabelFragment } from './fragments.generated';
+
+// Minimum contrast between the background and the text color
+const contrastThreshold = 2.5;
+
+// Guess the text color based on the background color
+const getTextColor = (background: string) =>
+  getContrastRatio(background, common.white) >= contrastThreshold
+    ? common.white // White on dark backgrounds
+    : common.black; // And black on light ones
+
+const _rgb = (color: Color) =>
+  'rgb(' + color.R + ',' + color.G + ',' + color.B + ')';
+
+// Create a style object from the label RGB colors
+const createStyle = (color: Color) => ({
+  backgroundColor: _rgb(color),
+  color: getTextColor(_rgb(color)),
+  borderBottomColor: darken(_rgb(color), 0.2),
+});
+
+const useStyles = makeStyles(theme => ({
+  label: {
+    ...theme.typography.body1,
+    padding: '8px',
+    margin: '0 8px',
+    fontSize: '0.9em',
+    borderRadius: '3px',
+    display: 'inline-block',
+    borderBottom: 'solid 1.5px',
+    verticalAlign: 'bottom',
+  },
+}));
+
+type Props = { label: LabelFragment };
+function LabelBullet({ label }: Props) {
+  const classes = useStyles();
+  return (
+    <span className={classes.label} style={createStyle(label.color)}></span>
+  );
+}
+
+export default LabelBullet;

webui/src/components/ValidLabels.tsx

 import React, { useState } from 'react';
 
 import CircularProgress from '@material-ui/core/CircularProgress';
+import { makeStyles } from '@material-ui/styles';
 
+import LabelBullet from './LabelBullet';
 import { useValidLabelsQuery } from './ValidLabelsQuery.generated';
 
+const useStyles = makeStyles({
+  list: {
+    listStyleType: 'none',
+    padding: 0,
+  },
+});
+
 const ValidLabels: React.FC = () => {
   const { loading, error, data } = useValidLabelsQuery();
   const [filter, setFilter] = useState('');
+  const classes = useStyles();
   if (loading) return <CircularProgress />;
   if (error) return <p>Error: {error}</p>;
   const labels = data?.repository?.validLabels.nodes.filter(
@@ -22,9 +32,12 @@ const ValidLabels: React.FC = () => {
         onChange={e => setFilter(e.target.value)}
         value={filter}
       />
-      <ul>
+      <ul className={classes.list}>
         {labels?.map(l => (
-          <li>{l.name}</li>
+          <li key={l.name}>
+            <LabelBullet label={l} />
+            {l.name}
+          </li>
         ))}
       </ul>
     </>