Add an ugly fragile hack to make the calendar fill the vertical space.

This is very fragile, but at least if it breaks it falls back to the
previous behaviour so it's not the end of the world if it does.

Fixes #16.

src/components/Calendar.tsx

@@ -61,7 +61,7 @@ class Calendar extends React.PureComponent {
 
   render() {
     return (
-      <div style={{width: '100%', height: 500}}>
+      <div style={{width: '100%', height: 'calc(100vh - 230px)', minHeight: 500}}>
         <BigCalendar
           defaultDate={new Date()}
           events={this.props.entries}