[analyzer] Add control flow arrows to the analyzer's HTML reports

This commit adds a very first version of this feature.
It is off by default and has to be turned on by checking the
corresponding box.  For this reason, HTML reports still keep
control notes (aka grey bubbles).

Further on, we plan on attaching arrows to events and having all arrows
not related to a currently selected event barely visible.  This will
help with reports where control flow goes back and forth (eg in loops).
Right now, it can get pretty crammed with all the arrows.

Differential Revision: https://reviews.llvm.org/D92639