3720: Clean up+skip Ruby SDK tests.

First I changed these to get them passing again.  With that done, I
added the skip.  We're not currently running these tests, and getting
them going the right way again (e.g., with run_test_server.py or
mocks) is out of scope for my current task.
3720: arvados-cli Gemfile refers to gemspec.

This change helps avoid loops where you have to have arvados-cli
installed to build arvados-cli.
3720: Refactor manifest parsing from API server to Ruby SDK.

To date, the API server has been parsing manifests and returning
parsed information in the files attribute.  That's convenient, but
causes performance problems in large Collections.  Profiling indicates
that most of the API server's response time is spent in the
as_api_response method; rendering the same information twice really
hurts.

This commits removes the API server's need to always parse manifests,
and moves the parsing code to the Ruby SDK.  Both the API server and
Workbench use this Gem to parse manifests on an as-needed basis.
This, combined with improvements to the new Keep::Manifest class, lays
the groundwork for future performance gains.
3720: Limit Workbench file rendering for large Collections.

Rendering too many files can cause rendering to take too long, and
there's not much point because it can really strain browsers too.
Arbitrarily cap rendering at 10,000 files.
Merge branch '3720-big-manifest-performance-wip'

Closes #3720, #3806.