Fallback to default profile photo if a chat does not have one

[xymeng16]

A default profile is made up of the first character of its first name and second name, with a colored background. But how does Telegram decide the color? Actually I don't want to check the official implement, instead let's just use some hash algorithm to select one from a hard-coded beautiful color lists:)

Also I think it would be better to pre-render the list so that we can save a lot of runtime rendering time.