More help text around the sidebar/general usability

[TylerFisher]

Seeing how our first users are using the sidebar, we should make some things clearer with some help text:

• Note on optional fields what will happen if you leave them blank
  • If slug is blank, we will create a slug based on the headline
  • If social media descriptors are blank, we will fill with search descriptors
  • Custom byline will override any authors. Also, you do not need to put "By" in custom byline.
  • Search description is also used around the site. (Maybe it should just be called "description"?)
• Put headline at the top instead of slug
• Move description below headline, if renamed
• Hide locale dropdown if there's only one locale available.