ts: Implement search

src/api/Backend.tsx

src/api/DummyBackend.tsx

src/api/api_v1_types.d.ts

src/components/CourseListing.tsx

src/components/DefaultLayout.tsx

src/components/Player.tsx

src/misc/PromiseHelpers.tsx

src/pages/courses.tsx

src/pages/internal/changelog.tsx

src/pages/search.tsx

src/styles/globals.scss