Flow topic subscription

watch/unwatch is handled by core watch api. no-js support
is handled by core action=watch, action=unwatch

Issues:

* Adding Topic namespace ( eg 520 ) to watchlist table, it automatically
  adds 521 for the same title

* Auto-subscribe is very annoying, If I don't want to subscribe to a topic,
  I need to manually unsubscribe after every reply/edit

Change-Id: Ibf00d44c74a0859053f1aad4f2a962eecc2e85db
Merge "Flow topic subscription"