[MODERATION] add user blocking API

- Follow up for: #540, #802
- Add API routes for user blocking from user and organization
perspective.
- The new routes have integration testing.
- The new model functions have unit tests.
- Actually quite boring to write and to read this pull request.

(cherry picked from commit f3afaf15c7)

modules/structs/moderation.go

@@ -0,0 +1,13 @@
+// Copyright 2023 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+import "time"
+
+// BlockedUser represents a blocked user.
+type BlockedUser struct {
+	BlockID int64 `json:"block_id"`
+	// swagger:strfmt date-time
+	Created time.Time `json:"created_at"`
+}