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>

routers/admin/users_test.go

@@ -87,3 +87,33 @@ func TestNewUserPost_MustChangePasswordFalse(t *testing.T) {
 	assert.Equal(t, email, u.Email)
 	assert.False(t, u.MustChangePassword)
 }
+
+func TestNewUserPost_InvalidEmail(t *testing.T) {
+
+	models.PrepareTestEnv(t)
+	ctx := test.MockContext(t, "admin/users/new")
+
+	u := models.AssertExistsAndLoadBean(t, &models.User{
+		IsAdmin: true,
+		ID:      2,
+	}).(*models.User)
+
+	ctx.User = u
+
+	username := "gitea"
+	email := "gitea@gitea.io\r\n"
+
+	form := auth.AdminCreateUserForm{
+		LoginType:          "local",
+		LoginName:          "local",
+		UserName:           username,
+		Email:              email,
+		Password:           "abc123ABC!=$",
+		SendNotify:         false,
+		MustChangePassword: false,
+	}
+
+	NewUserPost(ctx, form)
+
+	assert.NotEmpty(t, ctx.Flash.ErrorMsg)
+}