Remove "Display secure content only" option

[yuya-oc]

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Summary
Remove "Display secure content only" option.

allowDisplayingInsecureContent is removed in Electron 1.6 electron/electron@2e07803.
And it's no longer necessary to show http contents in https server.

Issue link
N/A

Test Cases

1. Post a HTTP link or an embedded image (![image](URL)) in a channel provided by HTTPS.
2. The image should appear.

Tested on Windows 10 and macOS 10.12.3.

Additional Notes
https://circleci.com/gh/yuya-oc/desktop/192#artifacts

[jasonblais]

Tested, works on Windows:

@jnugh would you be able to help test this on Linux?

[jnugh]

The images posted by @yuya-oc in the public test channel do not work... (But they don't render in the browser as well).

[Mattermost] Redirect from 'http://www.mattermost.org/wp-content/uploads/2016/03/logoHorizontal.png' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://pre-release.mattermost.com' is therefore not allowed access.

however other http image worked well.

[jasonblais]

ah yes, there's an issue on the master branch (and hence the pre-release instance) where images aren't loading properly..

in case you want to try and ensure it works on the desktop app, you can try on Mattermost v3.7.2 build here: https://rctesting.reddogsofwar.com

[jnugh]

Yeah, I checked on another instance. It's working fine ;)

[jasonblais]

Thanks @jnugh!

@yuya-oc I think we should be good to merge then

[yuya-oc]

Thanks all!