As part of an effort to make it clearer how the client uses Angular,
register UI components with `component()` instead of `directive()`.

Components in Angular 1.x [1] are essentially directives with "best
practices" defaults set (restricted to elements, isolated scope, bound
to controller) and deprecated/discouraged features of $compile
disallowed. These defaults/restrictions match how we are already using
Angular for most UI components.

[1] See https://docs.angularjs.org/guide/component