Markets

/cc @commuterjoy @matthew-andrews 

Adds a markets card at the top of a stream if a) Is an organisations stream and b) There is a company ticker symbol that is in more than 2 of the articles in a stream
(As a rough proxy for "This ticker is probably relevant to this stream)

Still stuff to do (mainly extracting out Stream.js into a separate component) - but wanna get it live behind feature flag to start with