Add informational modal to ZAP Search homepage

[TylerMatteo]

• Add a modal that users see when they visit ZAP Search that directs them to the new subscriptions page
• Designs can be found here but we don't need to follow designs exactly if there is an existing modal component in ZAP Search we can use. However, copy should be as seen in designs
• User can dismiss the modal and it doesn't reappear when navigating between pages in the app
• Modal has "Do not show this message again" checkbox. If checked, the user should not see the message again when they visit the app again in the same browser.
• Link to `/subscribe" page

[TylerMatteo]

Dependent on #1542