WebComment: improve comment reply handling

• When displaying the editor to submit a reply, use a markup that will let users insert inline comments in the quoted replies. This is done by using a block for each line instead of a single <div> for the complete quotation.

• Use <blockquote> tags instead of <div class="commentbox"> blocks, which are closer to how FCKeditor manage quotations (enabling users to insert standard quotes using the quotation button of the toolbar). Added style so that both techniques look the same.

• Improve washing of input comments so that they look better when used as reply in cases where FCKeditor cannot be loaded.

• Improve washing of comments sent via email to subscribers (HTML -> text conversion).

• Add new 'indent_block' parameter to function webmessage_mailutils.email_quoted_txt2html() to allow insertion of indentation markup at each line or each change of level of indentation. Add corresponding unit test.

• "Double-escape" input for the editor in order to correctly preserve the markup.