Filter a channel map from an input or output heading

[garethsb]

Edit: Merge #187 first, then rebase this branch onto master. Until then the diff includes that pull request. Done. This PR is now based off tip of master.

Summary

A match button on a channel-map heading sets the existing filters:

• An input shows the outputs that input can be routed to, including outputs with no routing constraints.
• An output shows the inputs that can be routed to it, and hides Unrouted when that output's routable inputs omit null.
• Unrouted shows the outputs that can be unrouted.

Test plan

• After rebasing onto master, yarn run lint-check and yarn test --watchAll=false
• Match an input, an output with constraints, an output with no constraints, and Unrouted

[lo-simon]

LGTM