BUX-931 Creates common event handler to handle banner close

[yli-branch]

modalBackground and closeButton both used the same onClick handling logic. This PR consolidates that logic into one helper function. Note: the variable name closeBanner is already in scope and taken, which is why I chose handleCloseButton. Other option I considered was handleCloseButtonClick - doing so might require us to consolidate the addEventListener('click', ...) logic into this helper function as well.

[yli-branch]

@JagadeeshKaricherla-branch What's the npm version I should be using when running npm install? I've updated my global settings to node: 14.15.0, npm: 8.11.0, and using these versions result in a large change to package-lock.json

[JagadeeshKaricherla-branch]

@yli-branch : I will do the merge and release for this later as we have a revert to do for another PR before merging this to prod.