always sanitize markdown

don't check if it's safe or not
move security js test
add cmp_tree, in case caja log can't be trusted

(spoiler: it can't)
use html-sanitizer instead of html-css

always scrub css (for now)
wrap caja.sanitizeAttribs to trust data-* attributes
check trust of data-attributes in sanitization
trust latex

If mathjax is insecure, we have big problems.
And we already trust mathjax in markdown cells,
so this is consistent.
sanitize untrusted HTML output

rather than checking is_safe
don't populate sanitized.safe by default

since it's potentially expensive.

walk nodes in is_safe
testing for sanitize
fix tagName comparison
don't use `result.safe` to communicate incomplete information
remove unused security warning
test style
sanitize CSS

rather than removing it entirely
default to allow_css = false
remove security.is_safe
remove struct-returning sanitize

only keep str-str sanitization
Add Trust Notebook to File menu
cleanup test_nbmanager

use class setUp / teardown instead of the same context manager in every test