Move 'mark_read' call out of 'get_context_data' and into 'get'.  Optimize number of queries to check for unread topics in forum.
Move 'mark_read' call out of 'get_context_data' and into 'get'.  Optimize number of queries to check for unread topics in forum.

Two reasons for moving the method call:
1. I wouldn't expect 'get_context_data' to have side effects.
2. It ensures that the view returns without error before marking the topic read

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