Move strip_ansi function to utils
Require jsonschema for nbformat, html and nbconvert tests
Skip some tests if nbformat not importable

Closes gh-5970
nbformat also requires jsonpointer
Add jsonschema & jsonpointer requirements for JS tests
Fix bootstrap location in package_data
Merge pull request #5971 from takluyver/jsonschema-test-dep

Allow tests to run without jsonschema