Allow setting dataURI as placeholder

[hahagu]

Allowing setting dataURI as a placeholder would allow custom implementations of placeholders.
This would partially solve the issue of using custom placeholder implementations for images, using things like blurhashes,
by allowing such to be loaded in via code on client side.

An alternative would be a pluggable system for generating such instead of useImage().

[Baroshem]

Closing in favor of #1307