Move can_unamend flag logic out of triggers

[exalate-issue-sync]

This ticket is to move the can_unamend code out of database triggers and into a mechanism invoked on the django code.

from the broader outline in the comment below, the tasks for this ticket are limited to the following for calculating can_unamend:

• test and gather metrics on the business logic prior to changes
• move the business logic out of the trigger and into django code
  • removing the trigger code with migration
  • creating new code in api that calculates and synchronizes the can_unamend flag
    * identify potential breakout test coverage work that might block this ticket.
• test and gather metrics on the business logic after changes

QA Notes

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DEV Notes

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Design

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See full ticket and images here: FECFILE-2798