Replace long press to edit mark with an edit button.

The previous code which triggered a mark edit on a long press over the
link caused all sorts of problems.  In particular it broke right
clicking to open in a new tab on desktop builds, and never worked
correctly on the iPad where the long press brought up the Safari menu
rather than the edit link menu.

Long press code removed and replaced with a button following the mark
text which activates the edit dialogue.  Don't really like the look or
layout but it'll do for now and ought to work everywhere.