Remove or replace Tether / Drop

[interactivellama]

Tether / Drop is old and hasn't been updated in two years. Recently, a team tried to render a tootlip on the server. Shallow renders don't work with Tether / Drop either (for instance in Jest testing).

Testing:

• Needs to work in Modals (z-index, render in inside portalMount)
• Needs to be able to flip based on viewport.
• Needs to render in Snapshots and server (no DOM environments)

See Dialog component for more details.

This might be a temporary solution: HubSpot/drop#155
This React port https://github.com/souporserious/react-popper of https://github.com/FezVrasta/popper.js might be a pretty robust solution.

Stale inactive WIP branch with partial implementation: #1022

[futuremint]

Here is my first attempt: master...futuremint:remove-tether-react-popper
That uses react-popper and changes Dialog to render both the target and the "popper", however while working on menu-dropdown this approach ended up being problematic...