Fixed user management related issues:

1) Unable to add a user when there are many users. The dialog went blank. Fixed.
 2) The error message did not have a close button. Fixed.
 3) When clicked on the add button, then the cursor will focus on the email box of
    the new row for convenience.
 4) When the dialog opens, the cursor will focus on the search text box.
 5) By default, the "User" role will be selected when adding a new user.
    Also removed an empty option in the roles dropdown.
 6) When the search filter is applied and we try to add an already existing user, the
    validation is done only on the filtered data and not on all the users' data. 

Fixes #5227

web/pgadmin/static/scss/_backgrid.overrides.scss

@@ -1,9 +1,12 @@
 .backgrid th, .backgrid td {
-  font-weight: normal!important;
   text-align: left;
   line-height: $line-height-base;
 }
 
+.backgrid td {
+  font-weight: normal!important;
+}
+
 .backgrid.table th.sortable > button {
   width: 100%;
   text-align: left;
@@ -112,6 +115,10 @@
     white-space: pre-wrap;
 }
 
+.backgrid > thead > th.renderable {
+  font-weight: bold;
+}
+
 table.backgrid {
   position: initial;
 }