add BoardToolbar component

src/components/board/BoardToolbar.tsx

@@ -0,0 +1,22 @@
+import * as React from 'react';
+import { FunctionComponent } from 'react';
+
+const BoardToolbar: FunctionComponent<{ visible?: boolean, style?: React.CSSProperties }> = ({ children, visible, style }) => {
+    if(visible === false) return <></>;
+    return (
+        <div style={style} className='toolbar'>
+            {children}
+            <style jsx>{`
+                .toolbar {
+                    display: flex;
+                    flex-direction: row;
+                    align-items: center;
+                    justify-content: space-between;
+                    align-self: stretch;
+                }
+            `}</style>
+        </div>
+    );
+}
+
+export default BoardToolbar;