Send by Email's "sendlink" does not support theming

The email for sending only the link is hardcoded to just the message followed by the link. There is no way to craft a custom e-mail with a boilerplate header or footer.

The attached patch adds theming support for 'sendlink' by adding three new theme hooks:

* print_mail_sendlink_html - Craft the message as an HTML message
* print_mail_sendlink_plain - Craft the message as a plaintext message
* print_mail_sendlink - Call the HTML or Plain theme functions depending on $format.

Themes can override these with print_mail_sendlink.tpl.php, print_mail_sendlink_html.tpl.php and print_mail_sendlink_plain.tpl.php template files.