Add optional links to individual gallery images

[richtabor]

This PR enables users to add individual links to gallery images within our gallery blocks (a common support request from WordPress.org).

There's still a few loose ends:

• Add a "Custom" value to the blocks' Link Settings PanelBody, which then reveals the imageLink UI
• The new link UI within the Stacked block is not working 100%
• The URLInput empty upon transforms, even if the current image's custom link is applied
• Carousel does not currently support links, do we remove the UI on Carousel - or work in support?

Preview:

[richtabor]

The Custom Link UI only displays if "Custom URL" is selected via Link Settings:

[richtabor]

This only works with the Stacked and Masonry blocks, as the carousel does not support links at all (currently).

[jrtashjian]

Looks good, 1 note but no change required.

[richtabor]

Looks ready to go!

[richtabor]

@jrtashjian Should we wait to merge this before the Jest tests?

[jrtashjian]

@richtabor we will need to implement integration tests to test this out, which are going to come at a later time. Let's go ahead and merge this one.

[benjaminluoma]

Is there a plan to add this to carousel blocks as well?

[richtabor]

Is there a plan to add this to carousel blocks as well?

Not immediately, though that's something I do want to look into. If I recall correctly, the big issue was figuring out how to properly navigate the carousel within the editor to add links - as currently, if you let off of the carousel (for example, to click into a URL field for an image), it resets the carousel instead of keeping it in place. If you have any ideas, I'm all ears :)