Tweak hidden topic behaviour

* Make it hidden by default
* Style it like current master - topic title followed by message about
topic being hidden
* Clicking it reveals the hidden topics

TODO:
I've styled closed topics with 0.4 opacity so they are more visually distinct
This reveals an issue with the toggle bottom in the top right corner. Clicking it
reveals closed topics.

Change-Id: I7406ebb47baab67cd1cba8ec5a078fcfb3ad923f
Merge "Tweak hidden topic behaviour" into frontend-rewrite