Improvements to CiviMail

A number of improvements need to be made to the CiviMail component so it is easier to use and behaves as expected,notably with respect to how emails do render in multiple clients. Most of these improvements are already existing, so this is mostly a matter of merging these in core.

Most notably:

• the embedded HTML editor must NOT load the civicrm CSS styles (cf. Parvez)
• the embedded ckeditor must be upgraded so it support the notion of non-editable element in email templates (cf. Parvez)
• a post-processing hook or other mechanism must be implemented to:
• remove the editable / non-editable tags (cf. Parvez)
• inline all CSS (cf. Eileen)
• remove HTML entities (cf. Lola)
• resize all images that do not confirm to their width/height attributes