Allow disabling of extensions

[afshin]

There are two fields in the page_config.json file that can allow control of which plugins load:

1. disabledExtensions for extensions that should not load at all.
2. deferredExtensions for extensions that do not load until they are required by something, irrespective of whether they set autostart to true.

The values for each are an array of strings.

• If an identical string match occurs between a config value and a package name (e.g., "@jupyterlab/apputils-extension"), then the entire package is disabled (or deferred).
• If the string value is compiled as a regular expression and tests positive against a package name (e.g., "disabledExtensions": ["@jupyterlab/apputils*$"]), then the entire package is disabled (or deferred).
• If an identical string match occurs between a config value and an individual plugin ID within a larger package (e.g., "disabledExtensions": ["@jupyterlab/apputils-extension:settings"]), then that specific plugin is disabled (or deferred). CC: @ivanov @ellisonbg
• If the string value is compiled as a regular expression and tests positive against an individual plugin ID within a larger package (e.g., "disabledExtensions": ["^@jupyterlab/apputils-extension:set.*$"]), then that specific plugin is disabled (or deferred).

Backward-incompatible API change

The MIME renderer extension interface no longer has a name field. Instead, it has an id field that follows the same semantic conventions as our other plugins. So for example, the name field of the Vega renderer has changed from name: 'vega' to id: '@jupyterlab/vega2-extension:factory'

[blink1073]

cc @gnestor, re: API change to mime extensions

[blink1073]

cc @lucbouchard1. In the next release you'd be able to replace extensions without manually removing them from the exports. Also, see #2936 which provides a script to validate and extract schemas and themes from extensions.

[blink1073]

cc @ivanov ^^

[blink1073]

Nice, thanks!