Add a `.gitallowed` file with a pattern for our Firebase API key. (#46)

This allows the `git-secrets` set of hooks to pass cleanly.

.gitallowed

@@ -0,0 +1,2 @@
+# Firebase API key
+AIzaSyDyAAcQeReDNXVUd_ZdkSzYgqIDJe-kPK8