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/components/ConversationsOverviewCard.js b/client/src/components/ConversationsOverviewCard.js
index b4206d5..6bb9dce 100644
--- a/client/src/components/ConversationsOverviewCard.js
+++ b/client/src/components/ConversationsOverviewCard.js
@@ -16,10 +16,10 @@
  * <https://www.gnu.org/licenses/>.
  */
 import { Card, CardActionArea, CardContent, CircularProgress, Typography } from '@mui/material';
+import { Conversation } from 'jami-web-common';
 import { useEffect, useState } from 'react';
 import { useNavigate, useParams } from 'react-router';
 
-import Conversation from '../../../model/Conversation';
 import authManager from '../AuthManager';
 
 export default function ConversationsOverviewCard(props) {