TinyMCE: fix the More and Next Page tags behavior:
- Append them inside top level <p> tags.
- If the caret is not in a top level <p>, create new paragraph after the current top level tag.
- Do not change placement when edited in the Text editor.
See #27378

git-svn-id: https://develop.svn.wordpress.org/trunk@27729 602fd350-edb4-49c9-b593-d223f7449a82