Clean up css, in particular, .col-md-3:nth-child(4n+1) { clear: left; } organizes entries on index.html.erb so that each 5th item (the first one of each new row) is aligned left, so that the rest of the page is layed out properly.
Add validations of field data in user.rb and listing.rb models.  ERROR: Add some CSS to have cleaner looking 'alerts' but not working everywhere -- ex: not working in listing#edit.
Working.
Install byebug debugger gem.
Return mistakenly deleted line.