Fixes #14639

This draft contains partially completed "Create a site" notifications. The currently working version creates a notification in the main activity which is shown to the user without a site. What's missing is:

• we need to set the delay when the notification is shown
• we need to schedule multiple notifications after various delays with various copys
• we need to update the texts of the notifications
• Unit tests for all the classes that could be unit tested

I think it's enough to do most of the changes in the CreateSitePushScheduler. We also don't need to cancel the notifications since all of them are only shown if there are no sites connected to the account.

To test:

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.