How-to block time input fields miss a label

[afercia]

Noticed while checking #10705

The "Time needed" hours and minutes fields need a label element, properly associated with for / id attributes.

Right now, they're unlabelled and announced just as "incrementable edit text" (that's the way number input fields are announced):

• ideally, these fields should be wrapped in a fieldset element
• "Time needed" should be the fieldset legend
• to avoid visual changes, the missing labels could be visually hidden with screen-reader-text, though visible labels are always preferable

Needs #10706