[MM-16146] When converting a bot to a user, set the role to system_user (#11361)

* when converting a bot to a user, set the role to system_user

* correct govet shadow error

* Change from inline error checking and fix shadow error

* Add SYSTEM_USER_ROLE_ID to the bot user model instead of updating the
role after the bot user is created

* modify UserFromBot test to include Roles field

model/bot_test.go

@@ -508,12 +508,14 @@ func TestUserFromBot(t *testing.T) {
 		Username:  "username",
 		Email:     "username@localhost",
 		FirstName: "display name",
+		Roles:     "system_user",
 	}, UserFromBot(bot1))
 	assert.Equal(t, &User{
 		Id:        bot2.UserId,
 		Username:  "username2",
 		Email:     "username2@localhost",
 		FirstName: "display name 2",
+		Roles:     "system_user",
 	}, UserFromBot(bot2))
 }