MM-24037 Adding getKnowUsers API endpoint (#14332)

* Adding getKnowUsers API endpoint

* Adding i18n strings

* Fixing golint errors

* Adding doc strings

* Remove debug line

* Updating app_iface

* Fixing gofmt

app/user.go

@@ -2083,3 +2083,10 @@ func (a *App) invalidateUserCacheAndPublish(userId string) {
 	message.Add("user", user)
 	a.Publish(message)
 }
+
+// GetKnownUsers returns the list of user ids of users with any direct
+// relationship with a user. That means any user sharing any channel, including
+// direct and group channels.
+func (a *App) GetKnownUsers(userID string) ([]string, *model.AppError) {
+	return a.Srv().Store.User().GetKnownUsers(userID)
+}