Improve sponsor filter UX and page layout

[mroderick]

Stacked on #2820 — review and merge that PR first.

What

Improves the admin sponsors filter form and page layout.

Changes

• Add page title (<title>) and <h1> heading
• Add breadcrumb with link back to Admin
• Replace raw inputs with labelled form fields in a styled fieldset
• Add validation error when chapter name is not found
• Style fieldset with Bootstrap border, background and rounded corners
• Fix pagination info tag to pluralize item names correctly
• Rename table heading from "Chapter(s)" to "Chapters"
• Remove duplicate pagination render

Screenshot

Testing

• bundle exec rspec spec/queries/sponsors_search_spec.rb
• bundle exec rspec spec/features/admin/filtering_sponsors_list_spec.rb