Stack name validation during creation to avoid name conflict

[deviantony]

Instead of allowing the user to create a stack with a name that is already used and display a warning after clicking on the create button, we should display a message close to the name input: "Stack name already in use".

This should be done in the stack creation view, when creating a stack via templates and when using the stack duplicate/migrate feature in the stack details view.

[yassirh]

Should the validation depend on the current endpoint or should it be global ?

[deviantony]

@yassirh right now, due to a technical limitation, stack name must be unique across all endpoints.

Related to #1347

[yassirh]

I will make a PR soon.

[yassirh]

Is there a service that returns the stacks over all the endpoints ? I have found StackService.stacks. however, I am not sure it loads everything.

[deviantony]

@yassirh yes, calling StackService.stacks(true, true, 0) will return all stacks.