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/common/tsconfig.json b/common/tsconfig.json
new file mode 100644
index 0000000..b03c254
--- /dev/null
+++ b/common/tsconfig.json
@@ -0,0 +1,9 @@
+{
+  "extends": "../tsconfig",
+  "compilerOptions": {
+    "declaration": true,
+    "outDir": "dist",
+    "rootDir": "src"
+  },
+  "include": ["src"]
+}