Add default banner for projects

[bvisness]

Right now project banners look like this:

This is sad. Instead of a gray rectangle, what if it looked like this:

Basically: use the "flowsnake" fractal that we are using for Unwind as a unique graphical fingerprint for each project. The size, rotation, and color of the pattern could be randomized based on the project ID. The exact sizes/colors can be tweaked later to look nice.

Ideally we wouldn't actually generate a unique image per project; we would just have the server emit some CSS that randomizes the properties we care about and renders the same SVG everywhere.

Flowsnake image:

[bvisness]

Asaf points out that we won't have a project ID to seed the pattern when we are creating a new project. I don't think that matters - people can all have the same pattern when they are creating a project, and then get something new when they hit save. It doesn't really matter. So, we can just seed with zero in that case.