fix: optimize ads with the mega menu

Resolves https://gitlab.eclipse.org/eclipsefdn/it/websites/eclipse.org/-/issues/396

The MR makes the following changes:

• Separates ad API-interacting code from UI code
• Adds unit tests for the API-interacting code
• Removes dependency upon jQuery from ads code
• Mega menu ads will render differently than the typical ad (introduces .eclipsefdn-mega-menu-promo-content class)
• Mega menu ads will be rendered when the mega menu submenu is shown; only renders once; can render different ads per submenu