[v4] Unify card and alert padding

Created by: KKSzymanowski

Currently .alert padding is defined as 15px and .card-header padding as 0.75rem 1.25rem. Because of this, the text isn't aligned vertically:

I don't know if this is intentional or not, but for me it doesn't look that good. Here is a live example with two possible fixes.

Is this something I should configure on my end or should I make a pull request to include it in the core?