Re-factor common Schematron validations

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

18F / fedramp-automation

Re-factor common Schematron validations #501