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/AuthManager.ts b/client/src/AuthManager.ts
index 2cd0279..29c5ffc 100644
--- a/client/src/AuthManager.ts
+++ b/client/src/AuthManager.ts
@@ -19,7 +19,7 @@
 /* eslint-disable no-undef */
 // TODO: This hides eslint errors for this file. This should be removed once this file is cleaned up.
 
-import { PromiseExecutor } from '../../model/util';
+import { PromiseExecutor } from 'jami-web-common';
 
 interface AuthManagerState {
   initialized: boolean;