Authenticate and subscribe to streaming after connection

Changes

• Authenticate via the post-connect message pleroma:authenticate, which is better than passing the access token in the query string, and way less hacky than the sec-websocket-protocol. The support has been added one and a half years ago ( pleroma!3864 (merged) ) so we can reasonably assume people's backend will support it.
• As a result, we have to use the unified streaming endpoint (because we need to authenticate first, and then subscribe to a topic)
• In the future, we can extend this to other timelines.

pleroma!3864 (merged)

requested review from @hj

mentioned in commit f6a1c4f2

merged