Make the model.views dict a dict of promises for views

Otherwise, there is a race condition where the model doesn't know about views until they are actually created.
Kill the layout manager
Fix scrolling for large outputs
Fix scroll/resize handle missing behavior.
Fix, prevent notebook from having its height set on pager display
Increase pager handle size.
add py3compat.buffer_to_bytes_py2

casts buffer-providing objects to bytes

needed for pickle.loads on buffers
cast buffers to bytes on Python 2 for pickle.loads

pickle.loads doesn't support the buffer interface on Python 2
Add warning to widget namespace import.
Address @carreau 's review comments
Add dark lower border to pager handle
Merge pull request #7107 from jdfreder/import_warning

Add warning to widget namespace import.
fix for mistume 0.5

default_features renamed to default_rules
update parse_ to output_
actually fix mistune ?
Merge pull request #7114 from Carreau/fix-master-mistune-0.5

fix for mistume 0.5
Bump mistune to 0.5 in notebook deps
Merge pull request #7093 from jasongrout/model-views

Make the model.views dict a dict of promises for views
Merge pull request #7115 from rgbkrk/bump_mistune

Bump mistune to 0.5 in notebook deps
Merge pull request #7105 from minrk/pickle-np-obj-py2

add py3compat.buffer_to_bytes_py2