Add email validity check (#13475)

* Improve error feedback for duplicate deploy keys

Instead of a generic HTTP 500 error page, a flash message is rendered
with the deploy key page template so inform the user that a key with the
intended title already exists.

* API returns 422 error when key with name exists

* Add email validity checking

Add email validity checking for the following routes:
[Web interface]
1. User registration
2. User creation by admin
3. Adding an email through user settings
[API]
1. POST /admin/users
2. PATCH /admin/users/:username
3. POST /user/emails

* Add further tests

* Add signup email tests

* Add email validity check for linking existing account

* Address PR comments

* Remove unneeded DB session

* Move email check to updateUser

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

integrations/api_admin_test.go

@@ -144,3 +144,22 @@ func TestAPIListUsersNonAdmin(t *testing.T) {
 	req := NewRequestf(t, "GET", "/api/v1/admin/users?token=%s", token)
 	session.MakeRequest(t, req, http.StatusForbidden)
 }
+
+func TestAPICreateUserInvalidEmail(t *testing.T) {
+	defer prepareTestEnv(t)()
+	adminUsername := "user1"
+	session := loginUser(t, adminUsername)
+	token := getTokenForLoggedInUser(t, session)
+	urlStr := fmt.Sprintf("/api/v1/admin/users?token=%s", token)
+	req := NewRequestWithValues(t, "POST", urlStr, map[string]string{
+		"email":                "invalid_email@domain.com\r\n",
+		"full_name":            "invalid user",
+		"login_name":           "invalidUser",
+		"must_change_password": "true",
+		"password":             "password",
+		"send_notify":          "true",
+		"source_id":            "0",
+		"username":             "invalidUser",
+	})
+	session.MakeRequest(t, req, http.StatusUnprocessableEntity)
+}