Show a warning if the installation image is beta software

[MrPotatoBobx]

TD;LR

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Since public beta will release soon(https://github.com/elementary/builds/pull/63), I would recommend adding some kind of warning that informs the user that the installation image that they are using is beta release software.

I think it should be shown as soon as the image is booted up (it may scare some users though at it might seem like an error)

or, we can show it when the end user clicks any of the options in the main installer (demo mode, clean install and advanced install)

Here is a concept I made in Krita (excuse my drawing skills)