Add lint to CI and fix errors (#853)

Adds lint to CI and fixes errors.
I'm not sure I'm fully proud of all the solutions there.
But there's no lint issues and the lint is being checked as part of the
CI.

---------

Co-authored-by: Yuri Astrakhan <yuriastrakhan@gmail.com>

src/components/Fieldset.tsx

@@ -1,6 +1,7 @@
 import React, { ReactElement } from 'react'
 import FieldDocLabel from './FieldDocLabel'
 import Doc from './Doc'
+import generateUniqueId from '../libs/document-uid';
 
 type FieldsetProps = {
   label?: string,
@@ -12,14 +13,12 @@ type FieldsetState = {
   showDoc: boolean
 };
 
-let IDX = 0;
-
 export default class Fieldset extends React.Component<FieldsetProps, FieldsetState> {
   _labelId: string;
 
   constructor (props: FieldsetProps) {
     super(props);
-    this._labelId = `fieldset_label_${(IDX++)}`;
+    this._labelId = generateUniqueId(`fieldset_label_`);
     this.state = {
       showDoc: false,
     }