More v4 endpoint additions

[samandmoore]

Summary

in #148 we discussed creating a v4 version of the /builds/:timestamp/split_registry endpoint that is used by the rails client and js client (transitively).

this PR adds that endpoint for future consumption. this is nice in that it allows all the clients to be consuming the same version of the split registry schema (v4) whenever they're updated.

this PR also introduces the experience_sampling_weight to the v4 endpoints.

NOTE: i moved the experience_sampling_weight loading behavior from the SplitRegistry model to the Rails.configuration in application.rb. This felt like a better way to share the behavior between the SplitRegistry-based and the non-SplitRegistry-based ways of getting at the list of applicable splits for a client (via app+version+timestamp or via build timestamp) cc @jmileham @aburgel the main result of this move is that the tests covering the tiny bit of behavior for validating the experience sampling weight are gone

/domain @Betterment/test_track_core /no-platform

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[nanda-prbot]

@samandmoore needs to incorporate feedback from @smudge. Bump when done.

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[samandmoore]

bump!

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