Add thread to collection when loading single thread.

When the thread that is being requested directly (through the url) isn't
part of the @discussion collection (only the first page of threads is
loaded initially, an old thread may not be present on the first page),
add it to the collection after it's loaded so that changes to the current
thread (upvotes, nr of comments...) are reflected in the sidebar.
Merge pull request #31 from mtyaka/thread-load-fix-2

Add thread to collection when loading single thread.