Fix WebSocket streaming API URL.

Small fix:

Current String.replace(Pleroma.Web.Endpoint.static_url(), "http", "ws" replaces all http occurrences in the string (which in a rare case can be present in a domain name)

• Use String.replace_leading instead of String.replace for getting WebSocket streaming API URL.

• Extract the login responsible for obtaining WebSocket URL into the corresponding Endpoint function.