Only show and use filters from enabled plugins

Rather than backtracking—starting with `FILTERS_NAMED` and then ripping out what shouldn't be there—it would likely be simpler and faster to start from the list of only the enabled plugins and then do the `append_update(...)` work (currently in the `FILTERS_NAMED` initializer) on just those. `all_plugins()` gives you a mapping of plugin name to Plugin (which has a `filters` attr), so that's a good place to start.