Auto-generate unique repository logo (identicons) (BB-10674)

For many projects, like code libraries, utilities, small projects and many other kinds, you have no graphical representation, no icon or logo for the project. And if you have many projects, they tend to include the same languages.

Therefore, the repository logos tend to be identical across most of a teams projects. Which makes them quite useless.

I propose (auto-generated) identicon logos for repositories without a custom icon set.

I think that could look awesome, and be very helpful.