Customize fielded search dropdown

Adds the bootstrap-select library, which allows us to turn <select> elements into fully customized dropdown menus, which Bootstrap does not allow natively.
Adds the SVG for the desired blue down-arrow in the fielded search dropdown and removes the old SVG arrow.
Re-triggers the JavaScript to load the bootstrap-select library when navigating the site through Turbolinks (similar to what had to be done with the image carousel).
Adds custom styling to the fielded search dropdown and better organizes _search_form.scss by component.
Removes the title attribute from the fielded search dropdown because the bootstrap-select library automatically renders the title as an initial default disabled option.

emory-libraries / dlp-lux