Prevent submitting parent forms when closing a modal

A weird issue noticed while working on #244 (closed): clicking the OK or close buttons inside of a Modal causes the modal to close, but also triggers submit events on all parent <form> elements.

This is a general issue with <button> tags: the type attribute is optional and defaults to type="submit", which is something we rarely want by default in a SPA. The React ESLint plugins includes a button-has-type rule, but I couldn't find one for Vue or for general JavaScript.

There are around 50 other cases of buttons without types in the frontend which could cause later issues with unexpected form validations.