Use native map when possible

Implemented on all modern browser.

There is a small behavior difference between $.map and Array.prototype.map
but we don't rely on it.

(jQuery flatten arrays and filters null-like)
Add REST API for retrieving, storing and updating config
Fix docstring, validate JSON on PUT
Apply JSON config updates recursively
Fix writing JSON on Python 2
Return updated config from PATCH requests
Add type parameter for contents GET requests
Expose and use get requests specifying type in the JS
Some fixes for the tree view
Allow specifying format when getting files from contents API
Fix various review comments
Rename get_model() to get()
Add test for /tree redirects
Actually pass URL params with API request
Add 48 pixel notebook icon
Merge pull request #6913 from takluyver/ipynb-icon-48

Add 48 pixel notebook icon
Put frontend config files in profile_foo/nbconfig/ subdir
Merge pull request #6735 from Carreau/native-map

Use native map when possible
Merge pull request #6694 from takluyver/config-rest-api

Add REST API for retrieving, storing and updating config
Merge pull request #6900 from takluyver/contents-api-get-as-type

Contents API get as type