Add gravatarImageUrlToGravatarImageUrl to utils + minor improvements

[maxme]

Main goal of this PR is to add a new gravatarImageUrlToGravatarImageUrl function.

Why adding this function here? Well, I think this can be a common pattern for other developers, not just for WordPress:

• developers store the avatar image url on their backend and returns the urls in their API.
• this API is used in mobile apps, thus they get the avatar image urls directly (instead of constructing them from email adresses).
• mobile apps might require to resize the avatar image to show them in different context (list view, full profile, etc), and to do that, they can leverage the gravatar backend by passing the right size parameter. Same for placeholder images, in some context we might want to show the default image, but in some other context we might want to provide the app's placeholder image (in that case, using the STATUS_404 default image might be a great use).

When we start writing documentation, we should provide examples of this one, but I'm not sure we should push for it.

This PR also:

• Fixes Add more query params to Gravatar URL generation #36: add more options to gravatarImageUrlToGravatarImageUrl and emailAddressToGravatarUrl
• Fixes Add missing options to default image #37: add missing robohash to the list of default image avatar
• Adds new test should cover new options and new functions

[maxme]

Note: I'm open to suggestions to rename gravatarImageUrlToGravatarImageUrl:

• rewriteGravatarUrl
• rewriteGravatarImageUrl
• rewriteGravatarUrlQueryParams
• rewriteGravatarImageUrlQueryParams seems more precise but is quite long.

[hamorillo]

❤️ Nice work @maxme! I love the amount of unit tests you added.

I left some comments, but they are all non-blocking.

[maxme]

@hamorillo, it should be good to go as I integrated all your comments, but better do a second pass.

[hamorillo]

❤️ Great job! 🚀