change

the extra column will get removed again.
if a database grows too large, all the extra rows for unsubscribed topics are a 
waste of space. We will now use a different mechanism to detect whether a topic is 'new':
For every person we will record the 'most recent topic' (s)he has reviewed on the 
review page. Any topic added after that is counted 'new' (indicated by topic-id, 
which is always ascending). A user can press a button on the review page to confirm that 
(s)he has seen the new additions, so they are no longer counted new. Any topic for which you are 
not explicitly subscribed and which is not new is counted 'unsubscribed'.

============================

A new addition: A user can SUBSCRIBE to a topic (was already so), or 
UNSUBSCRIBE to a topic (explicit: don't want to see).
Anything for which user is not explicitly sub- or unsubscribed, falls under 
'undecided'. 'Undecided' topics are separately available for review/subscription. 
Topics in a list get an extra button subscribe/unsubscribe (depending on state).

This is also dependent on 'database version management'.

==========================================================
Database extension
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See schema/checkschema.php, V0.2

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Functionality extension
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A user can SUBSCRIBE to a topic (was already so), or UNSUBSCRIBE to a topic
(explicit: don't want to see). Anything for which user is not explicitly sub- 
or unsubscribed, falls under 'undecided'. 'Undecided' topics are separately 
available for review/subscription (from the front page?). This allows a user
to quickly find really new topic in the tree, added by another user.

==========================================================
Interface extension
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Topics in a list get an extra button subscribe/unsubscribe (depending on state).
The front page will show a link to a list of all undecided topics as well as a 
list of all unsubscribed topics.
