Implement dynamic dialog height

The dialog currently has a hardcoded height of 450px. This causes the dialog to look overly tall with lots of empty whitespace when there are only a few search results, or when it only shows a single line success message.

• Changed panel to be stored on the class instance (this.panel), so we can reference it later to get its actual height.
• Updated getBodyHeight() to return this.panel.$element.outerHeight( true ) instead of the hardcoded 450. This tells OO.ui.ProcessDialog exactly how tall the content is.
• Added this.updateSize() to all the methods that change the visibility of the dialog's contents. This tells the dialog to animate and recalculate its dimensions smoothly every time its content updates.

Bug: T425117