html: Be careful not to strip too much

If there is content between to <style> elements, the content was previously
stripped by Format::safe_html function. This patch adjusts the regex to
strip tags such as <head>, <script> and <style> so that if content is found
between two of them, it will be preserved.
Merge pull request #686 from greezybacon/issue/674

html: Be careful not to strip too much

Reviewed-By: Peter Rotich <peter@osticket.com>