[dagit] Move WebSocket context

Summary:
Stacked on https://dagster.phacility.com/D9087.

• Remove WebSocketLink from AppProvider, move SubscriptionClient into WebSocketProvider.
• Modify context being provided by WebSocketProvider (which I renamed btw) so that it includes all the important stuff needed to set up WebSocket connections, notably connectionParams and websocketURI

Test Plan:
Add some headers to src/index, view runs in Dagit. Verify that WS connections now include those values as connection params on initialization.

Verify that WS connections are working as expected, including the connection status.

Reviewers: bengotow, prha, dgibson

Reviewed By: bengotow

Differential Revision: https://dagster.phacility.com/D9089