Fix Windows uninstaller not removing files correctly

[yuya-oc]

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Summary
Fix Windows uninstaller not removing files correctly

Multiple async tasks were not performed completely when running uninstaller.
So rearranged tasks with sequential order.
i.e. take care startup registry, finally call electron-squirrel-startup.

Issue link
#551

Test Cases

1. Install the app with the installer.
2. Enable "Start app on login" option.
3. Uninstall the app.
4. Files under C:\Users\%USERNAME%\AppData\Local\mattermost should be removed excepting .dead and Update.exe.
5. Registry key \HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run\Mattermost should be removed.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/280#artifacts/containers

[jasonblais]

@yuya-oc Works as expected. 👍

I guess the files in C:\Users\%USERNAME%\AppData\Roaming\Mattermost should still be there?

[yuya-oc]

Yeah, it contains user's configuration. So the uninstaller should not remove it without confirmation.

[jasonblais]

Sounds good, thanks!