GlAlert: Include necessary alignment styles in the component

Based on this review comment, I believe that the alignment styles for the GlAlert should be included in the component itself.

For example, I added gl-text-left to the component wrapper to align it all left.

---

Alternative: All components should ship isolated styles - it seems unlikely that we want a uniform component library that... isn't uniform

Alternative: Components should ship no styles, because they're intended to inherit styles from their consuming location - this seems unlikely, but implied by many components inheriting all CSS around them