Change xml parser

1. Switch to html5lib and lxml for handling XML (better error handling).
2. Update HTML files (e.g., replace '<td/>' with '<td></td>', turn '<caption>' inside '<figure>' into '<figcaption>') to get past stricter checking.
3. Turn the '&' in the <script src="..."> tag in `bootcamps/index.html` into '&amp;'.