Hide disabled connectors on list table filter dropdown

Introduce new filter: 'wp_stream_list_table_show_disabled_connectors'
to toggle the visibility.
Hide disabled connectors on list table filter dropdown

Introduce new filter: 'wp_stream_list_table_hide_disabled_connectors'
for toggling the visibility.
Hide records from disabled connectors on list table

Introduce 'wp_stream_list_table_hide_disabled_connectors_records' filter
Fix active connectors checking on post list table
Fix active connectors checking

Introduce WP_Stream_Settings::get_active_connectors()
Merge pull request #258 from x-team/issue-223

Remove disabled connectors from default queries and filter dropdowns.