Add lazy loading for covers in works merge tool

[RayBB]

Right now we load all covers on the merge page even if they're not visible. If we add lazy loading to them it will massively cut down on requests. Should be a pretty good first issue.

Describe the problem that you'd like solved

Proposal & Constraints

https://openlibrary.org/works/merge?records=OL21868175W,OL41495W

Additional context

Stakeholders

@cdrini