In preparation for the move away from AngularJS, replace
angular-sanitize with the DOMPurify library, which is a modern and
widely used HTML sanitization library with no framework dependency.

The sanitization logic is now encapsulated fully within the
`render-markdown` module which no longer takes a sanitization function
as an argument. That argument used to be necessary because
the angular-sanitize function had to be obtained via Angular's
dependency injection. DOMPurify can just be required as a module inside
render-markdown.js.

One improvement that is possible with DOMPurify but not done here is to
generate sanitized DOM nodes directly instead of having showdown
generate HTML, which is then parsed to DOM and sanitized, then converted
back to HTML and finally converted back to DOM by Angular.

Fixes #975