It is confusing to have a Babel plugin with an Angular-related name and
Angular-sounding annotations (`@ngInject`) in the code, even though
we're not using AngularJS. Although we're not using AngularJS any more,
we still have services that are instantiated by a dependency injection
container.  The container reads dependency names from a `$inject`
property on service functions/classes. This `$inject` property is added
by `babel-plugin-angularjs-annotate`.

This commit replaces the `angularjs-annotate` Babel plugin with one
maintained by us [1]. The new plugin provides only the functionality that we
need (eg. only processes explicitly annotated functions) and uses a more generic
`@inject` annotation.

[1] https://github.com/hypothesis/babel-plugin-inject-args