Charge manager: Don't allow setting the maximum current to 0 A.

[rtrbt]

[MattiasTF]

This can now only be enforced in the front-end. It cannot be enforced in the back-end anymore because the config is now split across different pages. If the maximum current is checked in the config’s validator or the value has a non-zero minimum value attached, attempting to save the “Charge management” page with no maximum current set on the “Chargers” page will fail with an unhelpful error message.