MM-34002: Improve AddUserToChannel (#17174)

* MM-34002: Improve AddUserToChannel

When we would add a user to a channel, we would
check whether the user is removed from that team or not.

During LDAP sync, this check is not required because the
team member would have just been created. Hence, we
pass a boolean flag to bypass the check.

And with that done, we can freely query the replica.

https://mattermost.atlassian.net/browse/MM-34002

```release-note
NONE
```

* Refactor code

* Rename a struct field

* fix double negative

app/plugin_api.go

@@ -478,27 +478,27 @@ func (api *PluginAPI) SearchPostsInTeamForUser(teamID string, userID string, sea
 }
 
 func (api *PluginAPI) AddChannelMember(channelID, userID string) (*model.ChannelMember, *model.AppError) {
-	// For now, don't allow overriding these via the plugin API.
-	userRequestorId := ""
-	postRootId := ""
-
 	channel, err := api.GetChannel(channelID)
 	if err != nil {
 		return nil, err
 	}
 
-	return api.app.AddChannelMember(userID, channel, userRequestorId, postRootId)
+	return api.app.AddChannelMember(userID, channel, ChannelMemberOpts{
+		// For now, don't allow overriding these via the plugin API.
+		UserRequestorID: "",
+		PostRootID:      "",
+	})
 }
 
-func (api *PluginAPI) AddUserToChannel(channelID, userID, asUserId string) (*model.ChannelMember, *model.AppError) {
-	postRootId := ""
-
+func (api *PluginAPI) AddUserToChannel(channelID, userID, asUserID string) (*model.ChannelMember, *model.AppError) {
 	channel, err := api.GetChannel(channelID)
 	if err != nil {
 		return nil, err
 	}
 
-	return api.app.AddChannelMember(userID, channel, asUserId, postRootId)
+	return api.app.AddChannelMember(userID, channel, ChannelMemberOpts{
+		UserRequestorID: asUserID,
+	})
 }
 
 func (api *PluginAPI) GetChannelMember(channelID, userID string) (*model.ChannelMember, *model.AppError) {