Be surgical about updating addon resources

[inaimathi]

Currently, each notebook loads all of the addon CSS and JS resources whenever any piece is changed.

Ideally, we'd end up loading only a minimal subset of the resources to make sure everything evaluates properly. This is going to involve some changes to the server-side addon system.

[inaimathi]

Low priority because

1. js resources are almost useless as of the parenscript cell additions
2. css resources should be relatively rare (they're really only called for when there's code that requires CSS changes across all notebooks. Single notebooks can do the same sort of stuff as parenscript cells, except inside of markup cells. Maybe even do a separate cl-css cell type?)