HH and MM fields in HowTo block do not allow 00

### How can we reproduce this behavior?
1. Install and activate Yoast SEO 10.0.1.
2. Start adding HowTo block: you cannot add `00` to hours and minutes fields in `Time needed`, only `0`, which feels counterintuitive.

<img width="788" alt="Screenshot 2019-03-28 at 12 41 04" src="https://user-images.githubusercontent.com/19681708/55155120-e014af00-5156-11e9-8f82-5fd04a83fbec.png">
