Fix expensive query on admin/users endpoint

In the admin/users endpoint. We check whether the delete user Button is rendered by checking whether there are groups where this user is a solo owner. This method has two issues

1. It checks for all the users separately, causing N+1 issues
2. It loads all the associated groups to the memory and checks the existence from ruby, this causes memory bloat

Low-hanging fruit to fix the issue is to offload the existence checks to the database so that entire groups need not be loaded to the ruby memory. Ref: https://www.speedshop.co/2019/01/10/three-activerecord-mistakes.html

For a user with 557 solo-owned groups

?> Benchmark.bmbm do |x|
?>   x.report("exists?") { User.find(1614863).solo_owned_groups.exists? }
?>   x.report("present?") { User.find(1614863).solo_owned_groups.present?}
[ gstg ] production> end
?>
Rehearsal --------------------------------------------
exists?    0.019400   0.003913   0.023313 (  0.454207)
present?   0.029882   0.000000   0.029882 (  0.214018)
----------------------------------- total: 0.053195sec

               user     system      total        real
exists?    0.012590   0.000277   0.012867 (  0.075993)
present?   0.028800   0.002822   0.031622 (  0.218427)