User module: Refactor user.admin.css file

In user.admin.css file, we have following code. Which can be optimised.

.permissions .permission {
  padding-left: 1.5em; /* LTR */
}
[dir="rtl"] .permissions .permission {
  padding-right: 1.5em;
  padding-left: 0;
}

Instead of having padding-left and padding-right, we can have padding-inline-start. So we will not need

[dir="rtl"] .permissions .permission {
  padding-right: 1.5em;
  padding-left: 0;
}