Refactor bootstrap_panel

#2468625: bootstrap_preprocess_bootstrap_panel() overwrites element id was originally the follow-up, but given how core now has both "details" and "fieldsets", this became even more complicated. Historically the "fieldset" theme hook was overridden to just use "bootstrap_panel". Now that 8.x has theme hook suggestions, it makes more sense to treat bootstrap_panel as an entirely separate theme hook rather than trying to use it as a "replacement".