Fix highlight nucleotides in airport view.

In an airport view we increase the size of nucleotides and color them red when
hovering and undo this when we stop hovering. This change does that. Previously
we always used pie.addLetters to draw a letter somewhere.
Decrease font size of airport view.

This makes everything look better in airport view with less overlap.
Mark long range interactions as dotted lines.

This makes it very clear which interactions are long range and does not force
the user to learn another button.
Add method to get all long range base pairs.

This will get all long range basepairs from the database, it returns an array
with nt1, nt2, family and crossing data.
Add crossing data to the page.

This json encodes the data and puts it in the page. In addition it adds some
help information about the crossing interactions which are displayed as dotted
lines.
Mark long range interactions as long range.

This merges the long range data into the interactions table and uses that
information to add a class to the interaction. This is how we mark interacitons
as long range in the drawing.
Update help modal.

This add some text about the airport view and long range interactions. In
addition it formats things a bit to make it more readable.