Adds search frontend and settings

[timurtripp]

This update:

• Adds a search modal which appears when clicking on the search icon in the top bar.
• Adds a new "Pages and search" tab to CU Boulder site settings (/admin/config/cu-boulder/pages). This tab contains settings accessible to Architect, Developer, and Site Manager:
  • The home page setting (moved from "General").
  • Options to enable site search, all of Colorado.edu search (default), both, or neither.
  • Configuration for the site search label, placeholder, and URL.
• Renames "Appearance" to "Appearance and layout" and alters the descriptions of menu items.
• Adds the Google Programmable Search Engine module, which allows creating custom search pages to use with site search.

CuBoulder/tiamat-theme#266

Sister PR in: tiamat-theme, ucb_site_configuration, tiamat10-profile