test installing nbextensions with symlink
Merge pull request #4778 from minrk/install-nbextensions

add APIs for installing notebook extensions
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
don't use combine option on ucs package

Caused problems with unicode.

It seems f9c32fcee337b6560ec31e25fba04cae6021b74a was actually the right fix for the original error, not faf51b858a8c5757944822ee1d75c11367e994f.

closes #5129
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
Merge pull request #5114 from minrk/empty-button

use non-breaking space for button with no description
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
fix a link that was broken
Merge pull request #5150 from ivanov/fix-fonnesbeck-link

fix a link that was broken
fix IPython typo, closes #5155
Merge pull request #5156 from ivanov/fix-5155

fix IPython typo, closes #5155
remove append_json reference

we never had such a method
Merge pull request #5095 from minrk/image-size

handle image size metadata in nbconvert html
Merge pull request #5159 from ivanov/oops-json

remove append_json reference
Simplify implementation of TemporaryWorkingDirectory.

Closes gh-5160
Merge pull request #5089 from rgbkrk/azure_out

Remove legacy azure nbmanager
Don't introspect __call__ for simple callables

Closes gh-5164
Added bottom margin to text widget