Implements block promoting to body (including position choosing based
upon likely tag position within parent) and block splitting where we're
only a single depth down from the body child.
Saves specifically the document element on output to HTML, since this
results in just the outer HTML being saved while not including the extra
XML tags which would show up before with the changes to force utf8
usage.
Adds a thin wrapper for DOMDocument to simplify and align usage within
all areas of BookStack.
Also means we move away from old depreacted mb_convert_encoding usage.
Closes#4638
