New behavior for hiding quoted text

In their hidden state, quotes will be limited in length, with a link to
extend them.  Only long quotes will be hidden; short ones will be
displayed in their entirety.  Note that the threshold for hiding a quote
is larger than the size of the hidden quote.  This ensures that there
will always be a noticable expansion of a quote when it is shown.  (This
also accounts for the fact that the height we measure includes padding.)

We can only measure the size of a quote once it is actually shown as
part of the document.  Notably, we can't figure out the sizes of quotes
in hidden emails.  Therefore, we also decide which quotes in an email
need to be hidden when the email is unhidden.  However, showing and then
hiding quotes can lead to content flashing, so we start all quotes off
in the hidden state, and remove this if they turn out to be short
enough.