Contact Info & Map Widget: improvements to Geocoding changes

[jeherve]

Changes proposed in this Pull Request:

• This PR extends on Contact Info & Map Widget: Remove Geocoding and add map to form Automattic/jetpack#12474 with some changes:

• Rely on esc_url_raw for remote call
• Clarify update process for the map, and coding standard changes
  • A few additional comments help understand when has_good_map is needed.
  • Use Yoda conditions, we must
• Simplify addition of query params to Google embed URL.
• Enqueue CSS on widgets page as well: this avoids that the map breaks out of the widget area layout when displayed.
• Display output from API request in widget area when error. This gives more information to users who entered an API key that is not correct for example, instead of just not displaying any map at all.

Testing instructions:

• They should be the same as Contact Info & Map Widget: Remove Geocoding and add map to form Automattic/jetpack#12474
• In addition to this, I'd recommend trying to input a wrong API key to see the error messages being displayed in the widget areas.
• Those error messages are only displayed in the old Widgets Screen, which is still a problem imo. In my opinion we should display those in the Customizer as well.

Proposed changelog entry for your changes:

• None