Store source/target causals on contribution itself, so they stay frozen once merged

[ChrisPenner]

Overview

Now that contribution diffs depend on the target causal and not just the BCA we need to track that on the contribution itself so we can freeze it once merged.

Implementation notes

• Store the current source_causal_id and target_causal_id on the contribution itself and update them when necessary
  Use these causals when computing diffs rather than the current state of the referenced branches.
• Carefully avoid updating these columns on contributions when the contribution is merged or is in the process of being merged.
• Replace contribution_diff_queue with a generic causal_diff_queue so we ensure no diff calculations get accidentally rolled together, and also so we can use it for generic namespace diffs

Test coverage

• Updated transcripts