Stop using hideFlash

What does this MR do and why?

For vue_merge_request_widget related files, we replace hideFlash by keeping track of created alerts and use their dismiss() methods.

For the rest, we replaced hideFlash by clicking on the close button in the alerts (JavaScript) or flash (Rails) elements.

On a follow-up we will remove the hideFlash code all together. We can't do it now, because addDismissFlashClickListener depends on it.

Screenshots or screen recordings

N/A

How to set up and validate locally

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