Add donor twitter handle meta field

[amberhinds]

This adds a Twitter handle field to the donors meta box in the donors custom post type. I'll be honest, I didn't test it anywhere 🤠 but considering it's a single meta field it's probably unlikely to break anything... You may want to test if you think the code doesn't look right. 😆

Checklist:

• My code is tested.
• My code is backward-compatible with WordPress 4.9 and PHP 7.0.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.

[joedolson]

Looks good.

[joedolson]

Removed whitespace at end of line & extra blank line.