proxy: show appropriate actions when logged in as management proxy

Cross-posted a user report here:

trying to set my voting proxy with my management proxy and I get this error:

Looks like you actually can't set voting proxy with management proxy, so this is probably a UI problem where this option shouldn't be available.

Azimuth only allows certain operations for each proxy:

So, Bridge should reflect which actions are actually available when logged in as Management Proxy

urbit / bridge