Make a couple simple improvements to some datagrids.

This makes a couple changes to the "Submitters" and "Groups" datagrids. To
start, I've renamed "Submitters" to "Users". I've also switched the order of
"Users" and "Groups". Finally, I've renamed the "Pending Reviews" column to be
"Open Review Requests".

Testing done: Looked at the changed datagrids.

Reviewed at https://reviews.reviewboard.org/r/5417/