Validation for existing slide name

[mgautam98]

While uploading new slide users are not notified if slide name is already taken. And multiple copies of same name exists. Due to this it creates bad user experience because
later users will not know what work they have done in which slide by looking at name.

Multiple slides with same name

Now it will be validated just like empty slide name.

[birm]

Yes, this is helpful! Thanks!