[MM-51236] - Add opt-in checkbox in self-hosted account creation page (#22709)

* [MM-51236] - Add opt-in checkbox in self-hosted account creation page

* add server impl

* fix lint and translations

* add tests

* improvents

* fix model

* fix mocks

* feedback impl

* update text color

* handle error

* fix tests

server/channels/einterfaces/cloud.go

@@ -47,4 +47,5 @@ type CloudInterface interface {
 	CheckCWSConnection(userId string) error
 
 	SelfServeDeleteWorkspace(userID string, deletionRequest *model.WorkspaceDeletionRequest) error
+	SubscribeToNewsletter(userID string, req *model.SubscribeNewsletterRequest) error
 }