Allow widget forms to work standalone, without being inside the customizer control context

[westonruter]

Remove forms dependency on customize-widgets; allow standalone forms to allow for Shortcake or frontend integrations

• Remove the passing of the WidgetControl as a property when constructing a Form; instead pass the model
  (which can be a Setting or a Value) and id_base directly, the latter of which should already be located
  on the Form prototype along with the config.
• Replace wp.customize.Widgets.formConstructor with wp.widgets.formConstructor
• Replace wp.customize.Widgets.Form with wp.widgets.Form
• Eliminate exporting all form configs to CustomizeJSWidgets.data.form_configs and instead attach to From prototypes.
• Fix initialization of Pages widget in how it amends the default config.
• Add Form.notifications
• Rename script handles to be more appropriate.
• Fix PHP warning for array to string conversion in Pages widget.
• Prevent RSS widget from showing error when feed URL is empty.
• Reduce duplicated code for rendering form templates.
• Breaking change: Converts/renames WP_JS_Widget::form_template() into wrapper method WP_JS_Widget::render_form_template_scripts() which outputs the script tags.
• Splits out form template contents into WP_JS_Widget::render_form_template().
• Eliminates extraneous id_base property for JS Form class, adding template_id form config which is then sourced from a new WP_JS_Widget::get_form_template_id(), which in turn is used by WP_JS_Widget::render_form_template_scripts() and is used in the JS Form#getTemplate method.
• Ensure that notifications for the setting (model) render even when the form is embedded outside the customizer.

Todo:

• Make sure that Notifications render properly when not in a control context.
• Allow JS widgets to work on the widgets admin screen.
• Restore ability for update method to be invoked. If sanitize returns WP_Error then return false.

This is a dependency for #11, to integrate with Shortcake.