Add avatar lightbox

[Sekhmet]

Closes #926

Changes:

• Add AvatarLightbox component that renders avatar and lightbox for that avatar with given image size.

[bonustrack]

@Sekhmet Isn't better to use https://ant.design/components/carousel/ for it and remove react-image-lightbox in favor of antd for single post too?

[Sekhmet]

I don't think this would be good candidate for lightbox, as it is designed as carousel.
Lightbox renders on top of everything by default and supports zoom and pan, which is helpful for big images. If we are going to switch to carousel we would have to implement those features ourselves.

[jm90m]

LGTM, I actually like the way lightbox looks, only problem with it is that there doesn't seem to be too much community support around it. But it looks good in mobile responsive.. I guess @bonustrack can decide :)

[jm90m]

Would it be better to use the default avatar username here? I guess it doesn't matter if it fails on that request then it goes to avatar default image anyways. So your call :)

[Sekhmet]

@steemconnect uses different avatar than we use as error fallback. I don't know whether there is username that has avatar set to that default avatar.

[bonustrack]

Wrong username got default avatar example https://img.busy.org/@xxxx123456 . We should not rely on @steemconnect avatar, at some point we will want to have the logo of SC as avatar.

[Sekhmet]

I've updated Avatar so it uses defaultImage instead of steemconnect as username if username is undefined.

[bonustrack]

@Sekhmet The image size should not be bigger than the original and also should keep aspect ratio. You can do this by changing the crop style like this:
http://img.busy.org/@blocktrades?crop=limit&s=800
http://img.busy.org/@fabien?crop=limit&s=800