Updates translations to use .org GlotPress

[noraconley]

Description of the Change

Updates translations to be able to utilize .org GlotPress.

• Updates text domain from mailchimp_i18n to mailchimp to match plugin slug
• Removes po files
• Removes text domain load functionality
• Updates translators: helper text

Closes #10

How to test the Change

• No visual regressions
• Translations will no longer be available internally and will rely on connection to translate.wordpress.org

Changelog Entry

Changed: moves translations to translate.wordpress.org GlotPress

Credits

@nateconley

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

@nateconley This looks good to me 👍🏻

I've gone through and found a few strings we weren't translating so added those. Also made some changes mostly for consistency around the use of esc_html__ vs __() or esc_html( __() ). If you could give a quick look at that commit to ensure I didn't mess anything up, that would be great: cae09ce

[noraconley]

@dkotter Thanks for getting those added in. I ran through the code, the admin page, and front end. Everything looks good.