Added branded emails for non html and GF Notifications

[DanielHudson2]

• Checks if an email sent to "wp_mail" has a content-type header for setting it to be HTML, if it already does it skips it (to handle already configured emails like woocommerce)
• If it doesn't already have html then it sets it, then wraps the content within a custom email template to give better formatting
• Specific scenario added for gravity forms, hooking into pre send email hook to wrap the content in the same custom email template
• filter orbit_branded_emails_disable can be used to completely disable this feature
• filter orbit_branded_emails_header_logo can be used to set a logo in the email header, if this isn't set then it falls back to a h1 containing the site name
• multiple filters in email-styles.php for controlling colours in email e.g. orbit_branded_emails_background_color
• Added template functions to include the templates which allows overriding within the theme for sites with more specific needs, added in their own class so that they can be reused in the future if needed for other features

[brettsmason]

Will add more comments tomorrow, but looking good so far. Just a few suggested changes.

[brettsmason]

@DanielHudson2 Looking great thanks!

A few points (only minor):

• The theme.json values are showing as variables, rather than the root hex code for me
• It would be nice to use WooCommerce colour values if available
• Replace Gravity Forms inline table colours with our own (maybe use the tint colour)
• There is a margin-left: 24px on the logo if one is set, which puts it out of alignment

The styling requirements might get more complicated, so perhaps we move styling to a class method and simply call the method in the email-styles.php file (probably return an array of the colour keys and values)?

[brettsmason]

We are looking good now, thanks @DanielHudson2 🎉