Load entries on Publish page asynchronously

[andrewminton]

Just a note for discussion as to whether the SBL is capable of being adapted to accommodate large data-sets from another section. The asynchronous part has been done for the section editor and I just wanted to raise this conversation for future development work.

[nitriques]

I think this discussion should be move to: https://github.com/DeuxHuitHuit/remote_selectbox 😄