Update options and UI improvements

Several minor changes:

1. Correct core update filter to `'allow_*_auto_core_updates'`, where `*` is `minor`, `major`, or `dev`
2. Convert `'update_control_core'` to a select, with valid options `minor`, `major`, and `dev`
3. Simplify `add_filter()` call: `add_filter( 'allow_' . $option['core'] . '_auto_core_updates', '__return_true', 1 );`
4. Add `'update_control_translation'` option
5. Convert `'update_control_active'` to a select
6. Fix broken jQuery handling of `disabled` attribute
7. Change jQuery script to add `readonly` attribute rather than `disabled` attribute, to preserve user settings for dependent options