[FIX] mail: don't autoescape mail subject

The mail template rendering system use "autoescape" for subject, body,
email_to, ...

Using autoescape is good for the body, but for the subject it means
variables insertions have to be marked as safe.

This commit add another environment without autoescaping enabled for the
subject field.

From https://tools.ietf.org/html/rfc2822:

  The "Subject:" and "Comments:" fields are unstructured fields as defined
  in section 2.2.1, and therefore may contain text or folding white space.

  2.2.1. Unstructured Header Field Bodies

  Some field bodies in this standard are defined simply as "unstructured"
  (which is specified below as any US-ASCII characters, except for CR and
  LF) with no further restrictions.

closes #10547

opw-659231
opw-666801
opw-665863