Add SSE to event forms

[benfrancis]

This is an update to the forms of Thing Descriptions to reflect the Server-Sent Events implementation in WebThingsIO#2863

Notes:

1. This adds a subprotocol member to event forms with a value of 'sse'
2. A top level form for properties or events is only added if the thing has properties or events respectively
3. I've removed the writeallproperties op from properties at the same time since I don't think it's actually supported in the API
4. I don't add a subprotocol member to proxied forms, I'm not sure whether or not this is the best thing to do