Set spellcheck attributes on form elements as appropriate

[jeblair]

Firefox does client side spell checking by default on textareas. This function can be controlled using an HTML attribute (spellcheck="true" / spellcheck="false") as described at:

http://developer.mozilla.org/en/docs/Controlling_spell_checking_in_HTML_forms

I'm attaching a patch that explicitly turns client-side spellcheck on for the Subject field in the compose window, as well as the TinyMCE editor. It also turns it off for the CC list and similar fields.

[jeblair]

Set spellcheck attribute as appropriate

[alec]

Applied in r1806.