Email input placeholder in subscription form

[max-ostapenko]

Hi,
we are running 2.3.9 and we think subscribers should see an email placeholder on a subscription page.
For example, adding just placeholder="email@example.com" will be a good addition to your redesign efforts.

Thanks.

[jbrooksuk]

Feel free to make a PR on the 2.4 branch :)

[max-ostapenko]

done #2560