When appending ?sort=fullname to /users/, ReviewBoard attempts to query and 

sort on 'fullname' which is not a correct field and causes an Error 500 to

occur. Although most manual sort queries will be discarded as improper, 

fullname is allowed as a sort field because it corresponds to a Column name on 

the page. This has been fixed by detecting the special case for 'fullname' in 

grids.py and if 'fullname' is in the sort query, it is removed and replaced 

with 'first_name' and 'last_name' which are its composite substrings.

Tested with Review Board 3.0.x by manually attempting ?sort=fullname and ?sort=-fullname on /users/, as well as sorting by fullname and username, and going back and forth between the fullname sort cases and username sort cases. I encountered no issues on my testing once the fix was applied.