Fix performance regression for ~b/~B searching. (closes #3743)

Fix performance regression for ~b/~B searching. (closes #3743)

In mutt_is_autoview(), changeset b58cdfacfb89 introduced a call to
rfc1524_mailcap_lookup() before checking if the MIME type should
be autoviewed based on the user's preferences. This caused a major
performance regression for ~b/~B searching.

Rearrange mutt_is_autoview() to check the user preferences first, then
search for a mailcap entry only if the MIME type should be autoviewed.

In order to preserve correct mime_lookup behavior, re-add a call to
mutt_check_lookup_list() before scanning the AutoViewList.