topic groups

[BrightonBTC]

Allow the user to group sets of topics and view them as 1 category.

Some topics are either basically synonyms of each other (eg. #btc and #bitcoin) or very closely related and would be better of grouped together to avoid potentially very long lists of topics.

The user can currently visit a topic page and click subscribe/unsubscribe, that should remain the same but clicking subscribe should offer the option to also 'add to category'.

Topics and categories should also be editable from the topic menu as shown in the attached image.