add line cost calculation modal in detail view

Adds a "How was this calculated?" prompt below the calculated line cost in detail view, which opens a modal dialog. The text in this dialog is incomplete - we should figure out what we want it to say. I'm using the "early-open-pull-request" workflow, in which I am starting work on a feature but don't really consider it done yet. If we even want this feature, let's figure out what we want it to say and how costs should be calculated and how those calculations should be presented to users so they can get feedback about their line routing decisions.

*note: untested, since I gave up after 5 minutes of futilely trying to install rails on windows. But it should work. Maybe someone with a working rails dev environment could test it.