[FIX] Apply top padding only to external link warning dialog

[arielj]

There was a regression with this change bda0871#diff-d9b11fef9fc96a7cf2707aa13652ab1309131d88dcdba70f00cd97fcccc25830R16 causing all dialogs to have extra top padding.

Causing this visual glitch:

This PR fixes that by applying the padding only to the external link warning dialog.

I also changed the hardcoded 20px to the css property that's used at the bottom so it's more theme-friendly.

(we should re-write this component in the future, it's using margin properties as padding)

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

lgtm