Move editor settings to a subform

The current editor's plugin settings on the filter format administration page are merged together into a single form element. This behaviour causes a variety of issues including the loss of #attached libraries set prior to the plugin settings being merged.

This is caused by array_merge(), where string keys that appear more than once overwrite the values of those that came before them.

Editor settings and those of any associated plugins should be moved to a subform in order to prevent them from interfering with the rest of the filter format form.