The HTML writer now supports `EndOfBlock`, `EndOfSection`, and
`EndOfDocument` for reference locations. EPUB and HTML slide
show formats are also affected by this change.
This works similarly to the markdown writer, but with special care
taken to skipping section divs with what regards to the block level.
The change also takes care to not modify the output if `EndOfDocument`
is used.
iOS chooses to render a number of Unicode entities,
including '↩', as big colorful emoji. This can be
defeated by appending Unicode
VARIATION SELECTOR-15'/'VARIATION SELECTOR-16'.
So we now append this character when escaping
strings, for both '↩' and '↔'.
If other characters prove problematic, they can
simply be added to needsVariationSelector.
Closes#5469.
