Clean up the Add gist form

[jure]

• The gist textbox should contain the label as a placeholder, and the label should be removed
• The copy at the top of the form is weird (identifier mentioned too many times, etc.)
• The login/signup should be moved from the Add gist page, if you're not signed in, to some other more appropriate location
• There's a bug with empty text in the gist textbox (error)