[meta] Fix Content Access UX issues.

This a meta issue for easy access to all issues that discusses the project's UX, to keep track of them and coordinate work on resolving them:

These four issues about the same problem: Misleading per node access settings.
Three of these my be closed as duplicates, but I keep all of them open to illustrate the same problem in slightly different ways:

• #1169084: usability of Per content node access control tab
• #1295686: Per-node access doesn't change Role Access Control text
• #2119155: Very Confusing UI under per Node Access Control
• #2895409: "Per node access settings" is confusing

Other UX issues:

• #836416: Unclutter (a.k.a. redesign) the 'Access control' tab
• #1192930: Per node override message: "Default access control for this content type is being overridden for this node."
• #3300360: Integration with the new per-content-type "Manage permissions" tab in Drupal core