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/ConversationView.tsx b/client/src/components/ConversationView.tsx
index e96b3f8..0a8e22d 100644
--- a/client/src/components/ConversationView.tsx
+++ b/client/src/components/ConversationView.tsx
@@ -16,9 +16,9 @@
  * <https://www.gnu.org/licenses/>.
  */
 import { Box, Stack, Typography } from '@mui/material';
+import { Conversation, Message } from 'jami-web-common';
 import { useCallback, useContext, useEffect, useState } from 'react';
 
-import Conversation, { Message } from '../../../model/Conversation';
 import { SocketContext } from '../contexts/Socket';
 import { useConversationQuery, useMessagesQuery, useSendMessageMutation } from '../services/Conversation';
 import ConversationAvatar from './ConversationAvatar';