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/SmallError.tsx

@@ -1,18 +1,22 @@
 import React from 'react'
+import { WithTranslation, withTranslation } from 'react-i18next';
 import './SmallError.scss';
 
 
-type SmallErrorProps = {
+type SmallErrorInternalProps = {
   children?: React.ReactNode
-};
+} & WithTranslation;
 
-
-export default class SmallError extends React.Component<SmallErrorProps> {
+class SmallErrorInternal extends React.Component<SmallErrorInternalProps> {
   render () {
+    const t = this.props.t;
     return (
       <div className="SmallError">
-        Error: {this.props.children}
+        {t("Error:")} {this.props.children}
       </div>
     );
   }
-}
+}
+
+const SmallError = withTranslation()(SmallErrorInternal);
+export default SmallError;