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/Messenger.tsx b/client/src/pages/Messenger.tsx
index fa10cfd..1fd2252 100644
--- a/client/src/pages/Messenger.tsx
+++ b/client/src/pages/Messenger.tsx
@@ -16,11 +16,10 @@
  * <https://www.gnu.org/licenses/>.
  */
 import { Stack } from '@mui/material';
+import { Contact, Conversation } from 'jami-web-common';
 import { useEffect, useState } from 'react';
 import { useParams } from 'react-router';
 
-import Contact from '../../../model/Contact';
-import Conversation from '../../../model/Conversation';
 import { useAppSelector } from '../../redux/hooks';
 import authManager from '../AuthManager';
 //import Sound from 'react-sound';