Editor: Restore isCleanNewPost selector

[aduth]

Previously: #8831
Related Slack conversation: https://wordpress.slack.com/archives/C02QB2JS7/p1536218969000100?thread_ts=1536160199.000100&cid=C02QB2JS7 (link requires registration)

This pull request seeks to restore an isCleanNewPost selector which was previously removed as part of the removal of its sole consumer within the editor package. It has surfaced that there were third-party integrations which depended upon this selector. As no alternative exists and it appears to be a useful selector for integrations, it has been restored here. A note has been included to indicate that it is not in use by the editor package itself.

Testing instructions:

There is no impact on the application.

Ensure unit tests pass:

npm run test-unit packages/editor/src/store/test/selectors.js

cc @ZebulanStanphill

[ZebulanStanphill]

This fixes the issue with Divi for me. 👍

[tofumatt]

I might actually use this over just using isEditedPostSaveable in #9608, but I'll amend the selector if that happens.

Before merging please add/restore the selector docs I (just recently!) removed in bbca724 for isCleanNewPost. 😄

[aduth]

As no alternative exists

I suppose it should be clarified that there is an alternative, which is simply to use the underlying isEditedPostDirty and isEditedPostNew to recreate the condition satisfying isCleanNewPost, as @tofumatt had done in #9608 .

[aduth]

Before merging please add/restore the selector docs I (just recently!) removed in bbca724 for isCleanNewPost. 😄

Thanks for the reminder. Updated in rebased cca95ab.

[tofumatt]

Update looks good.