* Squash four commits and claim.

Credit to @ikkemaniac in #25034, but taking this to sign CLA and get it landed. Four commit message were:

- add 'fill' switch on Alert Tab
- add 'fill' to Alert init model. Make default value 'true' not to break current design
- use newly created alert.fill when rendering graph
- add 'line' switch on Alert tab, add 'line' to Alert init model. Set default to 'true' not to break current design. Use newly created alert.line when rendering graph

Should close feature req #7258.

* Move alert toggle to Display tab.

* Move alertThreshold to PanelModel.options.

* Fix ThresholdMapper tests by adding options to each mocked panel.

* Update documentation for the new display option.

* Update docs with review feedback.

* Handle onRender with null panel in ThresholdMapper