[IMP] ir_qweb: remove the option 'only_date' introducted in cefc9ad8e3e3b0a14804da6c40bfd71e16d4175a. Now we can use widget='date' to do the same thing. That keeps the same behaviour that in the backend.

    <span t-field=model.datetime_field t-field-options="{'widget': 'date'}"/>
[FIX] perview button visible
[IMP][website_mail] improved template designer, added support for background color, font color and bootstrap buttons(inline css) in mail designer,removed other snippet, changed snippet according to new format,added theme selector in mail_designer, lodeded snippets dynamically based on theme, added font-awesome support in mail snippets, added one controller which convert font-awesome to icon