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/components/Header.js b/client/src/components/Header.js
index 165ff1b..637b529 100644
--- a/client/src/components/Header.js
+++ b/client/src/components/Header.js
@@ -1,6 +1,7 @@
-import { useState } from 'react';
 import { Box, Button, Menu, MenuItem } from '@mui/material';
+import { useState } from 'react';
 import { useNavigate, useParams } from 'react-router-dom';
+
 import authManager from '../AuthManager';
 
 export default function Header() {