Rework modal to support all client use cases

[raymondjacobson]

Reworks the modal to support use in protocol-dashboard & audius-client in all use cases. Those relevant PRs are:

audius-client: AudiusProject/audius-client#125
protocol-dashboard: AudiusProject/protocol-dashboard#16

There are three major changes in this PR:

1. Scroll counting is done internally within the modal now. Although this does break the "single counter" model that we share across the dapp to lock scroll count, I think it's ok for modals afaict. Once all the modals let go of their counter, scrolling is freed up. Works even with multiple modals.

2. Modals now create their own background / root context. One challenging issue I ran into when integrating this guy into all parts of the dapp is in cases where we have nested modals (e.g. stems modal inside edit track modal), the modals end up kind of canceling each other's backgrounds / root contexts out since they refer to the same dom node and both modals are mounted on the page at the same time. A unique modalKey similar to the scrubber pattern is added here (auto-assigned if not provided, but useful for debugging to provide).

3. Updates useClickOutside to respect an isException check. This is necessary to allow "useClickOutside" style dismisses to work in the case of nested modals. We never hit this issue in the dapp before because nested modals were always one antd modal and one stems modal so they never clashed in that sense.

[piazzatron]

Amazing 🎉
Only nits from me - I didn't check this on stage, but the code all looks great

[piazzatron]

Looks great, called out one thing in useGlobal