Note this commit also introduce a new {{d-popover}} component, example usage:
```hbs
{{#d-popover |state|}}
{{d-button label="foo.things" class="d-popover-trigger"}}
<div class="d-popover-content">
Some content
<div>
{{/d-popover}}
```
We cannot use any of the uppy mixins or core code, because
the code there is not shared with the wizard, and to move
it all to discourse-common would be a task almost equal
difficulty to taking the ring to Mordor.
Therefore, we can just use the uppy vendor libraries in the
wizard, and do a quick-n-dirty version of the uppy upload
code for the wizard-field-image uploader.