Remove jdenticon

Some of the jdenticons look really cool. Unfortunately many others do not look good, and we have no control over this.

So I think it's time to switch back to a simpler icon for users who do not have an avatar image. Let's do the following:

• Solid color background, chosen from our color palette by hashing the user's userId. This way, a given user will always get the same color, regardless of their displayname or avatar url etc etc.
• One letter initial in the middle of the icon. If the user has a displayname, take the first character of the displayname; otherwise take the first alphanumeric character from the user id.