Make the Scrollbar 16dips again (#12608)

BODGY: Controlsv2 changed the size of the scrollbars from 16dips to
12dips. This is harder for folks to hit with the mouse, and isn't
consistent with the rest of the scrollbars on the platform (as much
as they can be).

To work around this, we have to entirely copy the template for the
ScrollBar into our XAML file. We're then also re-defining
ScrollBarSize here to 16, so that the new template will pick up on
the new value.

This is kinda a pain, and we have to be careful to be sure to ingest
an updated version of the template any time we update MUX. The
latest Controlsv2 version of the template can be found at:
https://github.com/microsoft/microsoft-ui-xaml/blob/main/dev/CommonStyles/ScrollBar_themeresources.xaml#L218

We're also planning on making this adjustable in the future
(GH#9218), where we might need this anyways.

##### after, before:
![image](https://user-images.githubusercontent.com/18356694/156254464-1a9080f6-51ce-4619-b002-2a3c607cdf5f.png)

##### after overlayed on top of before
![image](https://user-images.githubusercontent.com/18356694/156254546-fccc3cee-12a3-4e1a-8fd7-7470f1ec93ad.png)

##### comparison
![image](https://user-images.githubusercontent.com/18356694/156257934-ec4ac840-c8ca-4fca-a848-08a32b1c55c3.png)

* reported originally in #12395
* upstream: https://github.com/microsoft/microsoft-ui-xaml/issues/6684
* closes an element of #12400

.github/actions/spelling/allow/allow.txt

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