Default avatar for "Ghost User"

Description

The ghost user should get a default avatar different from the "no avatar" icon other accounts get.

Proposal

It would be nice if one can clearly distinguish between a normal user who just hasn't set his avatar and the ghost user at first glance.

Links / references

Maybe just use a ghost emoji? http://emojipedia.org/ghost/

Documentation blurb

None necessary.

Use cases

When skimming merge requests, it's quite useful to see which comments are from current users and which are from deleted users.

Feature checklist

• Avatar icon
• "Ghost User" user avatar