This state hasn't been used since #5886. The nonce we display in our UI
is now from the background, rather than queried directly from the
front-end.
This also means we save making this network call each time a pending
transaction is added, and each time the transaction list is mounted.
The "global" action constants (the ones previously in `actions.js`)
have been moved to a separate module. This was necessary to avoid a
circular dependency in an upcoming change that was causing problems.
In general the "ducks" pattern of organizing Redux stores does result
in circular dependency problems. This is because reuse of actions
between reducers is encouraged, so it's not uncommon for two reducers
to want to reference an action from the other. Going forward we can
avoid this problem by moving action constants that are shared between
reducers into this shared module.
