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/ConversationList.tsx b/client/src/components/ConversationList.tsx
index f53aed8..b70001a 100644
--- a/client/src/components/ConversationList.tsx
+++ b/client/src/components/ConversationList.tsx
@@ -19,9 +19,9 @@
 import List from '@mui/material/List';
 import ListSubheader from '@mui/material/ListSubheader';
 import Typography from '@mui/material/Typography';
+import { Conversation } from 'jami-web-common';
 import { useEffect } from 'react';
 
-import Conversation from '../../../model/Conversation';
 import { useAppSelector } from '../../redux/hooks';
 import ConversationListItem from './ConversationListItem';