fixes some issues with gallery / embedded browse and javascript plugins

Fixes #454 #456 

(#454) Duplicate labels were being added for all "Select" forms in a gallery/brief view when a user clicked on a preview. `Blacklight.do_bookmark_toggle_behavior()` doesn't check if the form is already there but just adds a new one.

(#456) Truncation wasn't instantiated on gallery titles added in the embed browse docs