[dagit] Move WebSocket context
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
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