fix(route_manager history): move history update to after model save

[crankynetman]

We need to actually update the Entry change reason after we save the Entry model which then creates a history model for that change. Before, we were doing it in the serializer before the model is saved, which means that the query to the DB that looks for the appropriate historical_entry instance doesn't find anything because it's looking for the historical entry that hasn't been saved yet.

This is related to #138 and actually, I think, would have fixed it, but we came at it from a different (and frankly, I guess, wrong) angle. Because update_change_reason only edits a historical entry, not create a new one, but I was originally assuming that this function call was what created the historical entry in the first place. In #138 we "fixed" this issue by making sure that the database migrations went and edited all the history to have the default value which then means we technically were editing the comment for the wrong history entry.

[github-actions]

File	Coverage
All files	83%	✅
config/consumers.py	78%	✅
config/urls.py	69%	✅
config/settings/base.py	69%	✅
config/settings/local.py	73%	✅
scram/route_manager/admin.py	85%	✅
scram/route_manager/models.py	70%	✅
scram/route_manager/views.py	88%	✅
scram/route_manager/api/serializers.py	73%	✅
scram/route_manager/api/views.py	83%	✅
scram/shared/shared_code.py	56%	✅
scram/templates/403.html	91%	✅
scram/templates/404.html	91%	✅
scram/templates/base.html	99%	✅

Minimum allowed coverage is 50%

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