Sanitize app descriptions and render a subset of their HTML

Merged username-removed-26331 requested to merge grote/repomaker:20-sanitize-descriptions into master

This MR consists of two commits:

  • Sanitize HTML from remote app and repository descriptions and render what is left, including tests
  • Add simple TinyMCE editor to textareas that can include HTML
  • Fix the dependency problem with html5lib and cache python dependencies

Closes #20 (closed)