Fixed lint issues and missing documentation

internal/types/classifications.go

@@ -1,5 +1,9 @@
 package types
 
+// ClassificationCode is a code classifying what an opportunity buys. IsCPV
+// says the code is from the Common Procurement Vocabulary; Classification is
+// its description. ID is the database row, shared by every opportunity with
+// the same code.
 type ClassificationCode struct {
 	ID             int64  `db:"id"             json:"id"`
 	IsCPV          bool   `db:"is_cpv"         json:"is_cpv"`