This introduces djblets.http.responses.EventStreamHttpResponse, which

can stream events and data to a client by way of the Server-Sent Events

(SSE) standard.
This works by taking a generator or callable that yields dictionaries of

message data following the spec, allowing for app-specified IDs,

text-based payload data, event names, and retry intervals (for

disconnects) to be streamed to the client.
The response class takes charge of serializing this information for the

client, and can also support resuming of streamed events by intercepting

the SSE Last-Event-ID header and passing it to an event stream

callable.
It's important for consumers of this class to be mindful of how it's

used. Browsers have a limit of 6 concurrent HTTP/1.1 requests to the

same domain across all tabs, and a long-running SSE stream will take one

of those slots, potentially starving out other requests. This is far

less of a problem with HTTP/2.

Unit tests pass.
Tested with a makeshift EventSource implementation in JavaScript,

testing that events and all fields were successfully coming in and

processing as per spec.
Tested with curl and with GZip middleware turned on and off.