fix(og): N-03 - Swap misleading variable names

[Reinis-FRP]

Motivation

OZ identified the following issue:

The proposalHashes and assertionIds mapping names misleadingly reflect the
contents of one another (i.e., proposalHashes stores assertionId values, and
assertionIds stores proposalHash values).

Summary

This PR addresses this issue by swapping the names of the proposalHashes and assertionIds mappings, so that the variable names correspond to the values being stored.

Testing

Check a box to describe how you tested these changes and list the steps for reviewers to test.

• Ran end-to-end test, running the code as in production
• New unit tests created
• Existing tests adequate, no new tests required
• All existing tests pass
• Untested

[mrice32]

Woah, this one is really weird. LGTM! These all look swapped correctly.