Paella: Prevent video download. Disable context menu.

[miesgre]

There is no way to prevent the user from downloading an mp4 video, as this is a feature that is natively implemented by the browser, but we can make it a bit more complicated by preventing the context menu of the video from being shown.

This PR disable the context menu (configurable in config.json file)

Your pull request should…

• have a concise title
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• pass automated tests
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[ziegenberg]

Shift + Right-Click shows the context menu any ways: https://bugzilla.mozilla.org/show_bug.cgi?id=897379