Add headers and multiple subscription messages to Websocket source

[jbeisen]

Add headers and subscription_messages fields to the Websocket schema and
include them in the request. The console uses a new ArrayWidget
component to render this type of schema.

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The subscription messages can be set in sql like this:

    "subscription_messages.0" = 'first message',
    "subscription_messages.1" = 'second message',

Or if there's only 1, the old way:

    subscription_message = 'only message',

Or through the form like this:

[mwylde]

This will break existing websocket connections in your DB. Do you think it's worth keeping the old "subscription_message" around to provide backwards-compatibility?

[mwylde]

Looks great!