[FIX] mail: correctly format rejection email moderation content

When an incoming email on a channel is rejected, an email is sent to the
original author to inform him of the rejection. This email contains the
original email content added in a generic rejection email.

This original content was however considered as plaintext and escaped. Result
was quite hard to read. As content is sanitized we can safely append it to
the rejection email as valid html.

Task ID 2121319
PR #39978