Warn the user if the keyboard service is disabled (#8095)

## Summary of the Pull Request

![kb-service-disabled](https://user-images.githubusercontent.com/18356694/97578533-eb792d80-19be-11eb-9b13-b771327a72a0.png)

With this PR, the Terminal will check to make sure the "Touch, Keyboard and Handwriting Panel Service" is enabled at startup. If it isn't, then the Terminal won't be able to receive keyboard input (see #4448 and the 20 linked issues to that one).

## References

* See #4448 for more details

## PR Checklist
* [x] Closes #7886 
* [ ] Should this make #4448 not-open as well?
* [x] I work here
* [n/a] Tests added/passed
* [x] Docs: https://github.com/MicrosoftDocs/terminal/pull/168

## Validation Steps Performed

I manually set the service to "Disabled", restarted the machine, verified the dialog opens (and that I'm unable to type in the Terminal), then re-set the service to automatic and rebooted, and the dialog doesn't appear.

.github/actions/spell-check/dictionary/apis.txt

@@ -51,6 +51,7 @@ rfind
 roundf
 RSHIFT
 rx
+schandle
 serializer
 SIZENS
 spsc