Update function GetUnread

GetUnread didn't show unread discussions without comments. First fix (https://github.com/vanillaforums/vanilla/commit/1ab28953605778e1479ba9e518b10e367319d648) didn't respect permissions - thanks @peregrine for raising that issue (https://github.com/vanillaforums/vanilla/issues/1871)   
   
This fix includes unread discussions without comments and adds a permission check to the function