Fixing progress bar bug introduced by scrolling.
Changing position property to static for a more robust fix.
fix checkpoint filename
notebooks should always have one checkpoint

closes #4396
raise 404 on not found static file

master gives 403 due to empty string being outside of root
don't compute etags in static file handlers

etags skip cache timing, which we probably don't want.  If-Modified-Since is good enough for us.
allow `require("nbextensions/extname")` to load from IPYTHONDIR/nbextensions
Merge pull request #4427 from minrk/checkpoint-on-first-save

notebooks should always have one checkpoint

closes #4396
Merge pull request #4437 from minrk/etag

don't compute etags in static file handlers
fix and add shim for change introduce by #4195

comm merging renamed object_info_request to object_info
more detail changelog
Merge pull request #4436 from minrk/require-ext

allow `require("nbextensions/extname")` to load from IPYTHONDIR/nbextensions
remove compatibility layer
Merge pull request #4442 from Carreau/fix-oir

finish object_info_request/object_info rename in tooltip
Merge pull request #4435 from minrk/tornado-tweaks

raise 404 on not found static file
Merge pull request #4423 from damianavila/fix_prog

[Reveal] Fix progress bar and scrolling bug.
Added widget output area
Add widget subarea style
Added dependencies to page.html template and added widget.js to notebook.html
Added widget.js