DEV: Support for configurable list of extra jinja template directories.
DEV: Add IPython.html to the default template path.

This makes it possible to override a file with a template that also
inherits from that file.

For example, this makes it possible to override a single block of
notebook.html by creating a file called notebook.html that inherits from
templates/notebook.html.
DOC: Fix typo in comment.
DOC: Add whatsnew entry for userspace templates.
Merge pull request #6861 from quantopian/template_dir_option

DEV: Support for configurable list of extra jinja template directories.