Fix: Add email validation to newsletter subscription

[jordanmcdougall]

Description

Added a regular expression which validates the syntax of a valid email address on click of the Subscribe Now.

Fixes #106

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Tested against the following email addresses for valid addresses:

• email@example.com
• firstname.lastname@example.com
• email@subdomain.example.com
• firstname+lastname@example.com
• email@123.123.123.123
• 1234567890@example.com
• email@example-one.com
• email@example.name

Tested against the following email addresses for invalid addresses:

• email
• @example.com
• email @example.com
• email@ example.com
• email@example .com
• email@example.com

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules