Fix `CoreComponents` margin between inputs and labels

[NTurchi]

Description

This is a minor issue with the input component from the core_components.ex file (used in phx.gen.live) A margin top is added by default on inputs for the spacing between the label and the input.

However it is not removed if a label is not provided. Which causes alignment issue when combining the input component with other elements.

As suggested by José Valim, putting the margin on the label itself is more appropriate.

Changes

• Added a class attr to the label component
• Remove mt-2 from inputs in CoreComponents
• Add mb-2 to label with input

Closes #5832

before: margin on the input	after: margin on the label