Referrals: Switch to 2 months and make display of offer duration more flexible

[SergioEstevao]

| 📘 Part of: #2083 |
|:---:|

Fixes #

This PR updates the referrals code to support a more flexible way to define offer duration.
At the moment it changes the code to an hard coded 2 month duration but this will be resolved using IAP info in the future.

1	2	3

To test

1. Start the app from scratch
2. Ensure that you have the referrals FF enabled
3. Switch to the Profile view
4. Ensure that you have a Plus subscription (You can do it in settings -> developer)
5. Tap on the gift icon on the top
6. See if the guest pass duration is show correctly. ( It should be 2 months)
7. Tap on Close
8. Check that banner info for claiming a guest pass shows correctly
9. Tap on the banner for claiming guest pass
10. Check that the guest pass offer duration (2 months) and price after offer (39.99) is show correctly

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 7.73. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

Generated by 🚫 Danger

[bjtitus]

✅ Verified the guest pass time coming from the Gift button
✅ Verified the guest pass time + cost coming from the banner