Customize: Strip zero padding when setting month to prevent de-selection; add...

Customize: Strip zero padding when setting month to prevent de-selection; add zero-padding for `number` input fields when blurring.

Amends [42042].
See #42373.
Fixes #42392.

Built from https://develop.svn.wordpress.org/trunk@42104


git-svn-id: https://core.svn.wordpress.org/trunk@41933 1a063a9b-81f0-0310-95a4-ce76da25c4cd