<option selected disabled> should not be removeable

[chillu]

We're using chosen.js to present some inherited defaults, through <option selected disabled>.
These should show as readonly in the same way that browsers handle a native <select multiple>.
At the moment, chosen.js allows to remove them (through keyboard backspace or the delete icon),
which is not the expected behaviour.

Apart from removing the delete icon, they should also
be styled differently to communicate their state better - probably with a lower contrast border/text styling?

Note: This is different from tickets around marking the whole field disabled (<select disabled>),
or not allowing to choose options marked as<option disabled>. Looked for related tickets,
but couldn't find any direct matches (loosely related are #488, #248, f1e1c55, #67, #327).

The browser default styling and chosen.js sample can be reviewed here: http://jsfiddle.net/vUQVL/

[pfiller]

This is addressed in #638 and should be merged soon.

[pfiller]

#638 has been merged. This is now fixed in master.