fix(admin): remove duplicate display properties and use flex

[MaximeWillinger]

This PR fixes an issue on the app.blade.php layout.
Two different CSS display properties were applied with hidden and flex. This was ambiguous.

Also, the custom x-cloak behaviour was implemented using !block instead of !flex.
Using !flex here allows to make custom pages's root element full height using h-full.

[danharrin]

Because some people have custom themes, this was causing the page content not to display at all on the page when it was loading (as the !flex class didnt exist in their CSS).

I don't class it as a breaking change as we recommend people recompile their themes after updating, but so many people forget and will report this as their entire admin panel "broken" so I have reverted this PR.

Instead, I've removed the conditional class and hidden, and just made it flex by default.