Clean up and fix settings UI

• All plugin settings forms are displayed; it would be better if only the enabled plugins had their settings forms enabled.
• Plugin ordering/weighting has no effect and needs to be fixed.
• We should add configurable default settings for each plugin type -- or at the very least for vocabularies, since that has the most optionsMoved to #3150056: Allow defaults to be set for Vocabulary plugins?

Note that the display and ordering UI borrows heavily from the filter formatter UI, and the ordering/re-ordering of elements to enable, drag, and configure happens there as well.