Data layer restructuring (#997)

* Move query builders to sqlite package
* Add transaction system
* Wrap model resolvers in transaction
* Add error return value for StringSliceToIntSlice
* Update/refactor mutation resolvers
* Convert query builders
* Remove unused join types
* Add stash id unit tests
* Use WAL journal mode

pkg/models/sql.go

@@ -0,0 +1,17 @@
+package models
+
+import "database/sql"
+
+func NullString(v string) sql.NullString {
+	return sql.NullString{
+		String: v,
+		Valid:  true,
+	}
+}
+
+func NullInt64(v int64) sql.NullInt64 {
+	return sql.NullInt64{
+		Int64: v,
+		Valid: true,
+	}
+}