CRM-21264 - Fix broken regular expression

CRM-21264 - Fix broken regular expression

* Escape dot in token since it will be used in a regex (for good
  measure)
* Add `(?![\w-])` to ensure that `td` is not followed by other
  characters that would make it a different element.
* Instead of the first `.+?`, use `((?!</td>).)*` which matches zero or
  more characters but refuses to consume if it encounters `</td>`
* Escape curly braces (for good measure)
* Instead of the second `.+?`, use `.*?` so that it will match if there
  are zero characters. (Note: this change is really the crux of the
  issue, as described in CRM-21264).
* Complete then closing `</td>` (for good measure)