Add user blocking (#29028)

Fixes #17453

This PR adds the abbility to block a user from a personal account or
organization to restrict how the blocked user can interact with the
blocker. The docs explain what's the consequence of blocking a user.

Screenshots:


![grafik](https://github.com/go-gitea/gitea/assets/1666336/4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

services/user/block_test.go

@@ -0,0 +1,66 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package user
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/models/db"
+	"code.gitea.io/gitea/models/unittest"
+	user_model "code.gitea.io/gitea/models/user"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCanBlockUser(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	user4 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 4})
+	user29 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 29})
+	org3 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 3})
+
+	// Doer can't self block
+	assert.False(t, CanBlockUser(db.DefaultContext, user1, user2, user1))
+	// Blocker can't be blockee
+	assert.False(t, CanBlockUser(db.DefaultContext, user1, user2, user2))
+	// Can't block already blocked user
+	assert.False(t, CanBlockUser(db.DefaultContext, user1, user2, user29))
+	// Blockee can't be an organization
+	assert.False(t, CanBlockUser(db.DefaultContext, user1, user2, org3))
+	// Doer must be blocker or admin
+	assert.False(t, CanBlockUser(db.DefaultContext, user2, user4, user29))
+	// Organization can't block a member
+	assert.False(t, CanBlockUser(db.DefaultContext, user1, org3, user4))
+	// Doer must be organization owner or admin if blocker is an organization
+	assert.False(t, CanBlockUser(db.DefaultContext, user4, org3, user2))
+
+	assert.True(t, CanBlockUser(db.DefaultContext, user1, user2, user4))
+	assert.True(t, CanBlockUser(db.DefaultContext, user2, user2, user4))
+	assert.True(t, CanBlockUser(db.DefaultContext, user2, org3, user29))
+}
+
+func TestCanUnblockUser(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})
+	user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})
+	user28 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 28})
+	user29 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 29})
+	org17 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 17})
+
+	// Doer can't self unblock
+	assert.False(t, CanUnblockUser(db.DefaultContext, user1, user2, user1))
+	// Can't unblock not blocked user
+	assert.False(t, CanUnblockUser(db.DefaultContext, user1, user2, user28))
+	// Doer must be blocker or admin
+	assert.False(t, CanUnblockUser(db.DefaultContext, user28, user2, user29))
+	// Doer must be organization owner or admin if blocker is an organization
+	assert.False(t, CanUnblockUser(db.DefaultContext, user2, org17, user28))
+
+	assert.True(t, CanUnblockUser(db.DefaultContext, user1, user2, user29))
+	assert.True(t, CanUnblockUser(db.DefaultContext, user2, user2, user29))
+	assert.True(t, CanUnblockUser(db.DefaultContext, user1, org17, user28))
+}