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/package.json b/client/package.json
index f75fe9c..3285f86 100644
--- a/client/package.json
+++ b/client/package.json
@@ -39,6 +39,7 @@
     "emoji-picker-react": "^3.6.1",
     "framer-motion": "^7.3.5",
     "i18next": "^21.9.2",
+    "jami-web-common": "file:../common",
     "qrcode.react": "^3.1.0",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",