Fix bug when deleting first of two recently added blocks

[e-kolpakov]

Background: This PR fixes an issue with editing a library. Before the fix, due to differences between expected and actual DOM structure, XBlocks were sometimes added inside sibling XBlocks' wrapper elements.
JIRA ticket: https://openedx.atlassian.net/browse/OSPR-347
Discussions: Architecture discussed extensively on the wiki and in meetings, then the revised proposal was presented to the Arch Council on Oct. 21 and given thumbs up.
Sandbox URL: LMS at http://content-libraries.sandbox.opencraft.com/ and Studio at http://content-libraries.sandbox.opencraft.com:18010/
Partner information: 3rd party-hosted open edX instance, for an edX solutions client.

Testing instructions:

• Create a new empty library
• Add Blank Common Problem
• Add Checkboxes
• Delete the Blank Common Problem
• Checkboxes block should not disappear.

[openedx-webhooks]

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[openedx-webhooks]

Thanks for the pull request, @e-kolpakov! I've created OSPR-347 to keep track of it in JIRA. JIRA is a place for product owners to prioritize feature reviews by the engineering development teams.

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[openedx-webhooks]

Hello @e-kolpakov: We are unable to continue with review of your submission at this time. Please see the associated JIRA ticket for more explanation.

[antoviaque]

@e-kolpakov Closed the Jira ticket - this should have been a solutions jira ticket.

But to complete the final review in one place, I'm going to merge this into the current https://github.com/edx/edx-platform/pull/6459 MVP PR, along with the two other small fixes from @bradenmacdonald yesterday. Can you squash your commits? Thanks!