Add replacement gravatar configuration item

[willow-god]

Hello dear developer, since the support for gravatar in my area is not friendly, if you set an email address, gravatar will be automatically enabled, which will seriously slow down my website and cannot be replaced, so I would like to ask you to add an option to replace gravatar, or let me customize the avatar through the url.

My deployment method is docker, on the linux server.

Thank you for your project!

[codenyte]

@thomiceli You could use something like identicon or Jdenticon. Here's are a few suitable go libraries: https://github.com/rrivera/identicon, https://github.com/tsdtsdtsd/identicon