MDL-42711/MDL-42902 Fix the broken links in RSS feed block

If the <description> of the feed contains URLs longer than 30 characters, the
URL is split because of the break_up_long_words(). When combined with the
filter that converts URLs to links, this produces broken links.

The proper solution would be to improve break_up_long_words() so that it does
not modifies URLs at all. As a temporary solution for our purpose now is to
call format_text() prior to break_up_long_words() as it will not modify the
inner content of the <a> tag.

This should be fixed upstream. See also discussion at
https://moodle.org/mod/forum/discuss.php?d=34947