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
The error notification was being shown on creation of an intention. This
was as a result of #4572 and/or #4572 and has not been included in a
release.
This includes a fix, plus tests to try to prevent any further regression.