[ADAM-513] Remove ReferenceMappable trait.

[fnothaft]

Resolves (?) #513. Removes the ReferenceMappable trait, along with the ReferenceMappingContext. Also strips out the visualization code; @erictu will be moving this into a downstream repository.

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[fnothaft]

Rebased.

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[laserson]

Great work, @fnothaft! I'm happy to merge unless anyone has objections.

[erictu]

@fnothaft, should we strip out the webapp folder in adam-cli/src/main/webapp?

[tdanford]

I like it too, although I'd (minor quibble) suggest that we could at least drop the VizReads and TrackedLayout stuff as a separate commit (if not a completely separate PR)?

[fnothaft]

@erictu ah, forgot that! Will fix.

@tdanford I'll split that into a separate commit.

[fnothaft]

Rebased following the #606 merge.

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[laserson]

Thanks, @fnothaft! Happy to see ReferenceMappingContext go :)