Add filters/actions to customize widget visibility

See previous #167 #434 #6371.

It would be very helpful for us to be able to customize the list of criteria. Many of the previous proposals have been very broad and have been rejected.

If we could filter `widget_conditions_data`, that would be a good start:

Right now, we have:

wp_localize_script( 'widget-conditions', 'widget_conditions_data', $widget_conditions_data );

The easiest change would be to filter the output:

$widget_conditions_data = apply_filters( 'jetpack_widget_visibility_conditions_data',$ widget_conditions_data );
wp_localize_script( 'widget-conditions', 'widget_conditions_data', $widget_conditions_data );

For more control, it would be good to be able to include/exclude both (Category, Author, Tag, Date, Page, Taxonomy) and the values for each key.

Start by locating the `wp_localize_script` call for `widget-conditions` and tracing how `widget_conditions_data` is assembled. Compare the proposed filter with the requested category, author, tag, date, page, and taxonomy controls; done means the visibility criteria can be customized through the supported extension points.

Written by the indexing model from the issue text.