Configure topic creation with --topic.createEnabled

[ohjongsung]

Don't show the New button when --topic.createEnabled=false
The button is visible by default, no --topic.createEnabled is required

[ekoutanov]

We need to make sure that the create-enabled/disabled check is performed not only on the UI, but also in the controller code. I.e. the user shouldn't be allowed to bypass the UI and create the topic, if this has been disabled.

[ohjongsung]

--topic.createEnabled flag is also checked in the controller code.