#4454 Followup: "More" vendors dropdown

See https://github.com/internetarchive/openlibrary/pull/4454

BK has asked to avoid a large list of vendors overwhelming the page. Suggestion was to consolidate as is mentioned in this #4454.

Option to add a e.g. "other vendors" dropdown like goodreads
which shows more options.

We could do something similar for libraries as well (similar to goodreads) Turn Worldcat + LibraryLink into a dropdown button + also (on click) load ~3 nearby libraries from librarylink API into that list

### Describe the problem that you'd like solved

### Proposal & Constraints

### Additional context

We got a tentative :+1: from @bfalling 2021-01-22:
> “find it in other bookstores” and have it pop up a panel where any ol’ 3rd party link could be added, then that would be fine

### Stakeholders

@cdrini @bfalling