Add react-i18next for multi-language support (#917)

This is a rough start on adding react-i18next. I'll be working on adding
more translatable strings and translations in the coming days. I'm going
to need to wrap class components in HOCs, so let me know if there's
something I should be fixing before doing that. I'm thinking now to keep
the exported class names exactly the same, and rename the existing
classes by prefixing an `I` (for internal). For example:

```
export default class AppToolbar ...
```

becomes

```
class IAppToolbar ...
const AppToolbar = withTranslation()(IAppToolbar);
export default AppToolbar;
```

I'll be able to contribute Japanese strings (I've talked to a couple
people on my team and they'll be happy to help as well), so that's the
language I decided to go with in this PR.

Closes #746

---------

Co-authored-by: Ko Nagase <nagase@georepublic.co.jp>
Co-authored-by: Harel M <harel.mazor@gmail.com>

src/components/FieldId.tsx

@@ -13,7 +13,8 @@ type FieldIdProps = {
 
 export default class FieldId extends React.Component<FieldIdProps> {
   render() {
-    return <Block label={"ID"} fieldSpec={latest.layer.id}
+    return <Block label="ID" fieldSpec={latest.layer.id}
+
       data-wd-key={this.props.wdKey}
       error={this.props.error}
     >