`rpc` is used to define how to make a HTTP request/response messages.
This used to be called 'request' but the term is overused and sounded
like only one leg of the message, so the naming was moved to `rpc` a
while ago, this cleans up the places where we still used the old term.
Adds namespace support to the UI:
1. Namespace CRUD/management
2. Show Namespace in relevant areas (intentions, upstreams)
3. Main navigation bar improvements
4. Logic/integration to interact with a new `internal/acl/authorize` endpoint