Hide the main window on Mac auto-start

[yuya-oc]

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Summary
Hide the main window on Mac auto-start

Issue link
#583

Test Cases

1. Configure auto-start on Mac: https://support.apple.com/kb/PH25590?viewlocale=en_US
2. Select the "Hide" checkbox next to Mattermost.
3. Logout from Mac.
4. Login to Mac.
5. The app should launch automatically, but the window should not appear.

Additional Notes
Artifacts: https://circleci.com/gh/yuya-oc/desktop/966#artifacts

[yuya-oc]

I could not see significant problem. I merge this for now.