Add CTRL+SHIFT+MINUS as a shortcut for zooming out

[jasonblais]

I confirm (by marking "x" in the [ ] below):

• This is not a feature idea. Feature ideas go here: http://www.mattermost.org/feature-ideas/
• This is not a troubleshooting question. Troubleshooting questions go here: http://www.mattermost.org/troubleshoot/
• This is a product defect issue that can be reproduced using the steps listed below.
• I have read CONTRIBUTING.md.

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Summary

These are standard shorcuts that should work in any app. Original issue posted here.

Zoom in:

• CTRL+= [implemented]
• CTRL+SHIFT+= [implemented]

Zoom out:

• CTRL+MINUS [implemented]
• CTRL+SHIFT+MINUS

^Mac-equivalent shortcuts replaces CTRL with CMD.

If someone is familiar uses CTRL+SHIFT on the browser, then the experience appears broken on the desktop app since CTRL+SHIFT+MINUS doesn't work.

[ghost]

I'm taking this one. I will submit a PR within 24 hours.

[jasonblais]

Thanks @kytwb for your interest! Highly appreciated!

[ghost]

@jasonblais From what I read from other issues and guess from the implemented "Zoom In", the shortcut with SHIFT can't work with every keyboard layout, can it?

[yuya-oc]

Closed by #344