Update text-domain to be unique for this plugin

Each plugin should have a unique text domain.

I don't like abbreviations - and text domains should match the plugin slug - so even if Restrict Content Pro uses the rcp domain, I've gone with the full restrict-content-pro-mailchimp here.

Also fixed whitespace, a yoda condition, and changed "Options saved" to "Settings saved." to match the wording used in core.
Fix merge conflict