gh-trim-focus-input autofocus fix for iOS Safari

Closes #4446
- Mobile Safari doesn’t support the HTML5 `autofocus` attribute, but
also doesn’t play nice with jQuery’s `focus()` and `select()` methods.
- Mobile Safari wouldn’t automatically select the input field anyway,
so this PR simply removes the “jumping input fields” error without
changing any actual behaviour.
- Disallowing the programmatic selection of input fields (with select,
focus or similar methods) is apparently considered a feature rather
than a bug (ref http://bugs.jquery.com/ticket/12789)
- `autofocus` attribute is now only set on non-iOS devices