Make search work with category names (#4103)

core/base-service/examples.js

core/base-service/examples.spec.js

core/base-service/service-definitions.js

doc/TUTORIAL.md

frontend/lib/service-definitions/index.spec.ts

frontend/lib/service-definitions/index.ts

services/categories.js

services/github/github-downloads.service.js

services/github/github-last-commit.service.js

services/github/github-license.service.js

services/jira/jira-sprint.service.js

...vaadin-directory/vaadin-directory-rating-count.service.js

services/vaadin-directory/vaadin-directory-rating.service.js

...ices/vaadin-directory/vaadin-directory-version.service.js