Query builder (#10104)

* add query builder to facilitate dynamic SQL construction

* leverage query builder to refactor user store

This is partially setup work for MM-13120, but merged to master to avoid
further conflicts.

* fix unrelated unit test breakage

* documentation tweaks

* Apply suggestions from code review

Co-Authored-By: lieut-data <jesse.hallam@gmail.com>

* prefer comma separated case options to fallthrough

* vendor github.com/Masterminds/squirrel and deps

* switch to using github.com/Masterminds/squirrel

* rm querybuilder

vendor/github.com/Masterminds/squirrel/where.go

@@ -0,0 +1,30 @@
+package squirrel
+
+import (
+	"fmt"
+)
+
+type wherePart part
+
+func newWherePart(pred interface{}, args ...interface{}) Sqlizer {
+	return &wherePart{pred: pred, args: args}
+}
+
+func (p wherePart) ToSql() (sql string, args []interface{}, err error) {
+	switch pred := p.pred.(type) {
+	case nil:
+		// no-op
+	case rawSqlizer:
+		return pred.toSqlRaw()
+	case Sqlizer:
+		return pred.ToSql()
+	case map[string]interface{}:
+		return Eq(pred).ToSql()
+	case string:
+		sql = pred
+		args = p.args
+	default:
+		err = fmt.Errorf("expected string-keyed map or string, not %T", pred)
+	}
+	return
+}