remove base_kernel_url
band-aid for completion

Unlike @Carreau's attempt at using proper CodeMirror completion,
this is just a band-aid to get the existing completion back to working for 2.0.

This adds a keypress handler on the completer,
which then calls insert with the charCode of the key press event,
replacing the final `elif` branch of the keydown handler.

This cannot be done with keydown,
since keydown doesn't know what character is incoming,
only the hardware key that is struck.
add InlineBackend.print_figure_kwargs

removes special handling of 'quality', in favor of passing arbitrary kwargs to fig.canvas.print_figure.

This lets people override the occasionally problematic use of `bbox_inches='tight'`,
and any potential future issue that might be solved by adding/changing kwargs to print_figure.
remove application/pdf->pdf transform in javascript

We have the old transforms because they were added before we realized it was a bad idea. We should not add new ones.
handle 'application/pdf' in nbconvert

since we shouldn't add a 'pdf' shortcut
set default value from signature defaults in interact

If available, use the default value from the signature for the initial condition,
when using range/choice abbreviations.

Not affected:

- single-value abbreviations (`@interact(a=5)` sets `a=5`)
- explicit Widgets
remove no-op adding abbreviations from always-empty dict
Some template static_url tweaks

- don't try to include version in static_urls that don't point to files
- use full static_urls (with extension) for backbone and underscore, so version can be computed

closes #5141
Simplify implementation of TemporaryWorkingDirectory.

Closes gh-5160
Don't introspect __call__ for simple callables

Closes gh-5164
Added bottom margin to text widget
Added a class for RadioButtons container

with appropriate top padding...
fab css
Fixed radio button selector for new radio button class
Added msg_throttle sync=True widget traitlet
Added parentheses to emphasize order of op.
Fixed .. typo
remove mktemp usage

mktemp is insecure, by the time the generated filename is used someone
else might have modified it.
Merge pull request #5166 from juliantaylor/mktemp-remove

remove mktemp usage
Merge pull request #5163 from takluyver/i5160

Simplify implementation of TemporaryWorkingDirectory.