Homepage modal session logic: modal does not appear when expected on new sessions

[jilladams]

Session logic written in #11817 merged and was rebased into Josh's PR for #11535.

Using the Review instance for 11535 PR, I'm testing from http://a109d22ff3eebd5144235d7057fb577f.review.vetsgov-internal/, where both modal build and session logic are present.

Test cases

For each case, open http://a109d22ff3eebd5144235d7057fb577f.review.vetsgov-internal/, close the modal. Then:

🟢 Refresh page: Modal does not appear
🔴 Open a new browser tab and load http://a109d22ff3eebd5144235d7057fb577f.review.vetsgov-internal/: Modal does not appear
🔴 Quit browser. Open browser. Load http://a109d22ff3eebd5144235d7057fb577f.review.vetsgov-internal/: Modal does not appear

In order to get modal to reappear for the 2 failing cases, I have to first clear local storage. (localStorage.clear();).

Acceptance Criteria

• The soft-launch modal shows once per browser tab/window, regardless of how the visitor exits the modal (ie, dismisses the modal or follows the link to see the new homepage)
• Requires product review