Don't reset the readline completer after each prompt

This makes it impossible to setup a custom completer. The comment says that
"we must ensure that our completer is back in place," but I don't see why. All
that does it make it impossible to use a different completer. One can trick
IPython into not doing this by monkeypatching ip.has_readline to False, but
then certain features like multiline editing are no longer enabled.

See https://github.com/davidhalter/jedi/pull/321.
move raw cell inclusion to null.tpl

select based on raw_format metadata key if defined.
Default behavior is to include raw cells of unspecified format (consistent with existing behavior).
add raw_format to Exporter classes

sets their values in resources
test raw cell inclusion based on raw_format metadata
rename Raw Text to Raw NBConvert in menu
tweak global hide/show

make hierarchical rather than either/or.  Also, don't use the same class name for the outer and inner css classes.
give Raw Cells a placeholder

and make sure cell_type is defined *before* calling TextCell.apply
clarify name of 'default' cell toolbar
allow cell toolbar presets to apply only to specific cell types
add raw cell toolbar preset
add dialog to raw cell toolbar
set cell_type properly first-class in the Cell object

- in toJSON
- don't clobber existing value in Cell constructor
fix Python mime-type
propagate raw_mimetype to nbconvert
Making the cell margin 0 all around.

Also moves some css/less code to the outputarea.less.
Adjusting padding of output subareas and adding @code_padding.
Tabs to spaces in variables.less.
More tabs to spaces on less files.
tweak raw placeholder
Cleaning up indentation and spacing of less files touched.