Encouraging correct usage of keys

When rendering items in a list React recommends using a key to keep track of the item between re-rendering (docs, guide). In 10.x we covered over some issues when incorrectly using keys. 11.x did not mask incorrect key usage. In this release we have:

• 👩‍⚕ Restored the old (still somewhat broken) behaviour when incorrectly using keys
• 👷‍♀ Logging a development only warning when we detect incorrect key usage
• 👩‍🏫 Created a guide to help you using key with react-beautiful-dnd

Closes #1274 #1278 #917

Other

• Internal linting for useLayoutEffect (#1281)