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.

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

src/components/InputString.tsx

@@ -46,7 +46,7 @@ export default class InputString extends React.Component<InputStringProps, Input
     let tag;
     let classes;
 
-    if(!!this.props.multi) {
+    if(this.props.multi) {
       tag = "textarea"
       classes = [
         "maputnik-string",
@@ -60,14 +60,14 @@ export default class InputString extends React.Component<InputStringProps, Input
       ]
     }
 
-    if(!!this.props.disabled) {
+    if(this.props.disabled) {
       classes.push("maputnik-string--disabled");
     }
 
     return React.createElement(tag, {
       "aria-label": this.props["aria-label"],
       "data-wd-key": this.props["data-wd-key"],
-      spellCheck: this.props.hasOwnProperty("spellCheck") ? this.props.spellCheck : !(tag === "input"),
+      spellCheck: Object.prototype.hasOwnProperty.call(this.props, "spellCheck") ? this.props.spellCheck : !(tag === "input"),
       disabled: this.props.disabled,
       className: classes.join(" "),
       style: this.props.style,