Migration of jsx files to tsx 2 (#850)

This is to continue the work of migrating all the jsx files into tsx
files.
The MO is basically described here: #848.

About 7 files to go...

src/components/Collapser.tsx

@@ -0,0 +1,19 @@
+import React from 'react'
+import {MdArrowDropDown, MdArrowDropUp} from 'react-icons/md'
+
+type CollapserProps = {
+  isCollapsed: boolean
+  style?: object
+};
+
+export default class Collapser extends React.Component<CollapserProps> {
+  render() {
+    const iconStyle = {
+      width: 20,
+      height: 20,
+      ...this.props.style,
+    }
+    return this.props.isCollapsed ? <MdArrowDropUp style={iconStyle}/> : <MdArrowDropDown style={iconStyle} />
+  }
+}
+