Map Block: Persist Center Point

[jeffersonrabb]

Changes proposed in this Pull Request:

This PR makes a few changes to the calculation and persistence of the Map block center point. The block determines the center point based on the map points that have been set. When the block initializes, the map "travels" to correct position and zoom, which causes a dramatic and unnecessary animation. This PR persists the true center point of the map, allowing the map to initialize in that position, avoiding the animation.

This PR also adds a bonus feature: in a map with no points defined, the editor can determine the center point by dragging, and this position will be respected when the map renders.

If any points have been chosen, dragging is disabled. This is done to avoid the false expectation that the new position will be respected, since the block will replace it with the calculated center point based on the available points.

Based on Automattic/wp-calypso#28811.

Testing instructions:

• On master, add three Map blocks to a page with 0 points, 1 point, and 2 points.
• Switch to fix/map-block-center-point, verify that the blocks remain valid and work as expected.
• Create a new post and add Map blocks to it: 1) no points defined, left in original position (SF, CA) 2) no points defined, dragged somewhere else 3) one point defined 4) multiple points defined
• Verify the positions of all four remain intact after saving and reloading the editor, and that there is no initial animation.
• Verify the position of all four are identical in the front end, and that there is no initial animation.
• Verify the first two can be dragged, and the second two cannot.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello jeffersonrabb! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D37943-code before merging this PR. Thank you!

[jetpackbot]

	Warnings
⚠️	"Proposed changelog entry" is missing for this PR. Please include any meaningful changes

This is an automated check which relies on PULL_REQUEST_TEMPLATE. We encourage you to follow that template as it helps Jetpack maintainers do their job. If you think 'Testing instructions' or 'Proposed changelog entry' are not needed for your PR - please explain why you think so. Thanks for cooperation 🤖

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[Copons]

This works great and is an awesome improvement over the previous behaviour!
I've left a couple of minor comments, but the PR is pretty much good to go! ✨

[matticbot]

jeffersonrabb, Your synced wpcom patch D37943-code has been updated.

[jeherve]

This tests well for me! 🚢

[jeherve]

r202023-wpcom