Add `common` subproject containing shared files

Move classes in `model` to their own `common` package.
Now, the client and server can import `common` as a library.
This is the first step to eventually migrate all the source code at the root of the
project to an `old-server` package.

GitLab: #55
Change-Id: I4b7a52e80171d9c3399416ab524bcdd6915ac540
diff --git a/client/src/pages/ServerConfiguration.tsx b/client/src/pages/ServerConfiguration.tsx
index 99e7547..56d06a8 100644
--- a/client/src/pages/ServerConfiguration.tsx
+++ b/client/src/pages/ServerConfiguration.tsx
@@ -17,10 +17,10 @@
  */
 import CircularProgress from '@mui/material/CircularProgress';
 import Container from '@mui/material/Container';
+import { Account } from 'jami-web-common';
 import { useEffect, useState } from 'react';
 import { useParams } from 'react-router-dom';
 
-import Account from '../../../model/Account';
 import authManager from '../AuthManager';
 import AccountPreferences from '../components/AccountPreferences';
 import Header from '../components/Header';