Only projects have descriptions; repositories no longer keep their own #138

Shipped: only projects have descriptions. Deploy #352 (88b51ede) moved every reader and writer to the project's description, and Deploy #353 (4e351794) dropped repos.description and projects.repo_description. Checked live: the API's repositories have no description, every project kept its own, link previews show the project's, and reading and searching repositories still work.