Add configuration to make email a required field

[NickHu]

This patch adds a parameter to make requiring an email address configurable.

Could probably be improved by:

• write tests
• using HTML5 form validation
• not using a nasty regex substitute to remove (required) from the placeholder (or internationalised equivalent)

[posativ]

Thanks for the contribution. Email verification is not easy and usually requires a validation email being sent. I am okay with the current implementation though.