Missing class def for TimeoutError.
Adding comment about this fix.
Merge pull request #5265 from ellisonbg/timeout-error

Missing class def for TimeoutError
Adding security.js with 1st attempt at is_safe.
Don't render insecure Markdown and show warning.
Display safe HTML+SVG even if untrusted, but don't set trusted=1.
Adding first round of security tests of is_safe.
use google-caja for sanitization
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