Add bulk operations to subscriber view

I added a basic implementation and one operation "Delete subscriber" for example.

We need to discuss what other operations are needed. Maybe new operations can be done in other tasks so as not to block the current one.

1) My main concern is that the fix adds a lot of lines of code - and already this module has way too much code to easily maintain. I would like to check if all this code is really necessary, or if there could be a simpler solution. I looked at Core, and I see that Node.php is similar to what you did, but Media and User entities are simpler.

@adamps, thanks for the comments. I reviewed the code and agree that it could have been made simpler. I have updated the current version of the code.

Maybe we don't need to alter existing views in the update hook - some sites might even prefer not to have a bulk action.

I think existing users should get the new changes as well as new users. I removed these changes to make it simpler.

2) It would be great to have a test of the bulk function.

Added.