core: helper: make an API for dashboard and header bar widgets

### Current behaviour

1. Widgets need to be independently and directly designed to pull from relevant services, instead of defining widgets as part of the services themselves
2. There is no way for external services (e.g. Extensions or cloud) to provide widgets
3. If all widgets are defined independently, it's more likely we'll end up with inconsistent interfaces for doing similar things

### Expected or desired behaviour

- [ ] Expand the helper service to look for endpoints presenting json (?) definitions of dashboard and header bar widgets, which can then be included in the selectors for each
- [ ] Allow some pre-defined templates
- icon + label + value
- a URL field for an endpoint that gets displayed in an iframe
- [ ] Include a link field, for clicking through to the underlying service page
- [ ] Provide an API that gets the pinned state of a header bar widget, and allows pinning one
- [ ] Provide components and/or interface recommendations for common features
- pin icons in a dashboard widget, making use of the pinned state API for header bar widgets
- an info icon for hovering to get more information about a given component

### Prerequisites

- [x] I have checked to make sure that a similar request has not already been filed or fixed.