ajax_modal: Combine trigger and preserve selectors

...in one event handler registration, so if something matches both
trigger and preserve selectors, it still only gets one event handler
call, not two. Fixes #772

Needs to be backported to a 5.0.2 release as a bugfix.
ajax_modal: ensure preserve selectors properly scoped.

since you can list multiple selectors, they each need to be manually listed as
scoped inside the dialog, just string-prefixing the ancestor wasn't going to
work.

Plus better comment docs.
Merge pull request #775 from projectblacklight/ajax_modal_double_submit

ajax_modal: Combine trigger and preserve selectors