Manage element types without reloading the whole corpus list

This triggers stale read issues (see backend#411 (closed)) that are not fixable (the corpus list must stay on replicas)

The frontend implementation is a bit naive, and does not maintain a state (simply reloading the whole corpus list).

We should:

• have a store to manage the corpus types
• initialize that with the corpus list
• update the state on addition / edition of types