Sort imports from all files

Add "prettier" script in `package.json` to check if all files are
compliant.

Sort all imports and format all files by running `npm run lint -- --fix`
followed by `npm run prettier -- --write`.

Gitlab #29

Change-Id: Id80402d1344c3bc611f66a46cffeee5f2c375bd8
diff --git a/client/src/themes/ThemeDemonstrator.js b/client/src/themes/ThemeDemonstrator.js
index 6f12209..d989569 100644
--- a/client/src/themes/ThemeDemonstrator.js
+++ b/client/src/themes/ThemeDemonstrator.js
@@ -1,13 +1,14 @@
 import { Button, Stack, Switch, ThemeProvider, Typography } from '@mui/material';
+
 import {
-  CancelPictureButton,
-  EditPictureButton,
-  UploadPictureButton,
-  TakePictureButton,
-  InfoButton,
-  TipButton,
   BackButton,
+  CancelPictureButton,
   CloseButton,
+  EditPictureButton,
+  InfoButton,
+  TakePictureButton,
+  TipButton,
+  UploadPictureButton,
 } from '../components/buttons';
 import { NickNameInput, PasswordInput, RegularInput, UsernameInput } from '../components/inputs';
 import defaultTheme from './default';