* DEV: remove duplicate code in button component template
* DEV: refactor is-loading state of d-button component
Before this change on initialisation `forceDisabled` is set `false` and then might change to `undefined` - depending on the use of the button component. This change ensures a boolean value for `forceDisabled`.
The added test works with and without the new change. The test is added as it represents the default use case for most buttons.