Created by: hnrch02
Fixes #15136 (closed). Closes #15345. Closes #15314. Refs #14724, #14927.
Effectively reverts #14724 but still fixes #13816 (closed). The backdrop will now again be appended to the body and not the .modal container.
Also includes a workaround for when an opening accordion makes the modal overflow to minimize jumping on the dialog. (It's more of a proof of concept, I understand if we take it out before merging.)
/cc @cvrebert @fat @XhmikosR
Sorry it took so long guys.