Revert "Put an insert element button between and above each element."

This reverts commit 757be4229348c5d32972ec0b780457194305e7e7.

The new new Element insert Button has some serious design flaws, that have to be solved before we put this into master.
Fixes creation of external pages.

The presence validation of Page#urlname only should be triggered on update action.
Merge pull request #577 from magiclabs/fix/creating-external-pages

Fixes creation of external pages.
Use pre release gem in README Installation notes