Improved import UX

Pull request for #60 

Changes: 

- [x] Condenses URL/GitHub/GDocs import into single URL field
- [x] Takes all other import parameters (delimiter, name, etc) and puts them in a preview/save pane at the top of a new project, and makes the values live-update the views
- [x] There's an explicit save step. The project wont autosave as much as before.

There are some things I'd still like to do, but they could be post-merge.

- [ ] We should intelligently choose the initial value for the delimiter based on file extension/mime type/sniffing. Same for text delimiter.
- [ ] Reintroduce an import from gdocs button, to simplify obtaining the URL.
- [ ] Hide fields that have no effect, e.g. delimiter for gdocs import
- [ ] Check a URL exists (and has CORS headers if necessary) as soon as the user enters it.
- [ ] It would be nice to properly support data which doesn't have a header row.