Re-factor common Schematron validations (source: 18F/fedramp-automation: 501)

[danielnaab]

Original issue: https://github.com/18F/fedramp-automation/issues/501

Extended Description SSP, SAP, SAR, and POAM instance documents all have common (FedRAMP) metadata and back-matter Schematron validations. These should be placed in a separate document so that they can be used in each of the respective instance document validation Schematron documents.

Acceptance Criteria

• [ ] All Schematron assertion messages are declarative statements which affirm the positive test outcome.
• [ ] All Schematron assertion diagnostic messages are declarative statements which explain the negative test outcome.
• [ ] The Schematron code has no assertion failures when validated using src/validations/styleguides/sch.sch using the basic phase.
• [ ] XSpec unit tests for positive and negative Schematron assertion outcomes accompany all Schematron assertions (where feasible).
• [ ] Story tasks are completed.

Story Tasks

• [ ] Identify common validations currently in ssp.sch.
• [ ] Create a new Schematron document containing the common validations.
• [ ] Determine how the common validations should be incorporated by reference in the instance document related Schematron.
• [ ] Alter the instance document related Schematron to incorporate the common validations.
• [ ] Test using command line and UI.

Definition of Done

• [ ] Acceptance criteria met
• [ ] Unit test coverage of our code > 95%
• [ ] Automated code quality checks passed
• [ ] Security reviewed and reported
• [ ] Reviewed against plain language guidelines
• [ ] Code must be self-documenting
• [ ] No local tech debt
• [ ] Load/performance tests passed – needs to be created/automated
• [ ] Documentation updated
• [ ] Architectural Decision Record completed as necessary for significant design choices
• [ ] PR reviewed & approved
• [ ] Source code merged