Display error message when typing invalid username

Description: Display an error message when using invalid characters after trying to change a username in the Contacts section of the Admin dashboard . Problem: Currently the Admin user tries to change the username of a contact but doesn't know about the characters that are not permitted (uppercase,blank spaces, underscore ). When the Admin users tries to edit the username and uses one of those invalid characters, the user clicks on Save and the system just reverts back to the previous username, returning no error or warning about the restriction. Benefit: This would warn the Admin user on the characters allowed when editing or creating a username and not lead them to think that it is a bug in the system causing the change not to save. Note: there was a previous post on this but it's on Completed status, although the issue persists : https://app.sleekplan.com/feedback/412