Add lcsh subjects

This is intended to handle #43.

I discussed LCSH with @zemkat, who says that the ordering of LCSH terms (e.g. "Kittens in art" and "Kittens--Behavior") has meaning for some but not all users.  She says that while some users definitely care about order (or at least "primary" versus "secondary" subjects), for many users the order matters less as linked data becomes more common.

Right now, this code does not make any attempt to respect user-selected subject order.

I am using @awead's lcsh_suggest engine to do the actual term lookup and Select2 for the UI.  The database caches submitted subjects (shared across all users), but they are serialized by name, not id.  If we start getting folks submitting thousands of subjects, we'll need to revisit this.

There are no migrations in this pull request, but you will need to run bundle install and restart your rails server to see the changes.