Clear input when the user closes the modal

[nlhkabu]

How to reproduce:

• Log in as a package maintainer
• Go to the 'projects' page and select a project
• Go to a release
• Click on the 'delete release' button
• Enter something into the confirmation input in the modal
• Close the modal (either with 'cancel' or with the cross at the top of the modal)
• Click on 'delete release' button again

What happens

The second time the user launches the modal, the text from the first time is still in the input

What should happen

The second time the user launches the modal, the input should be empty

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