feat: store the check content digest

[csmig]

Addresses #898
Resolves #899 #900 #901

Database migration

• add columns lowCount, mediumCount, highCount to the tables revision and current_rev and to the view v_current_rev. This supports counting the rules in a Revision by severity, which is a prerequisite for calculating the scoring formula in Calculate and display the Weighted Value Index (WVI) #898.
• alter column revision.ruleCount to become a generated column whose value is highCount + mediumCount + lowCount (Calculate and display the Weighted Value Index (WVI) #898)
• update existing records in revision and current_rev to have values for the new counting columns (Calculate and display the Weighted Value Index (WVI) #898
• create table check_content to store check digests, with primary key ccId (check content identifier) (Store STIG Check content with an associated content hash and ID  #899)
• alter tables check and rule to add column ccId (Store STIG Check content with an associated content hash and ID  #899)
• insert records into check_content (Store STIG Check content with an associated content hash and ID  #899)
• update the values of the ccId columns in check and rule (Store STIG Check content with an associated content hash and ID  #899)
• update the ccId columns in check and rule to have an index and foreign key constraint (Store STIG Check content with an associated content hash and ID  #899)
• drop column check.content (Store STIG Check content with an associated content hash and ID  #899)

API

• modify queries that return check content to join with the check_content table (Update queries that fetch STIG Rule check content #900)
• modify the service handler for POST /stigs to remove any existing revision data before proceeding (Change STIG-import behavior to overwrite #901)
• modify the service handler for POST /stigs to support rule counts by severity (Calculate and display the Weighted Value Index (WVI) #898)
• modify the service handler for POST /stigs to support the check_content table and ccId columns (Store STIG Check content with an associated content hash and ID  #899)

Newman

• tests updated to verify correct behavior