Adds tracking to the Add Site buttons

[emilylaguna]

Project: #17503

Description

This adds a new event when the Add Site Alert is displayed that includes a source of where it was tapped from. This also modifies the enhancedSiteCreationAccessed event to include a source.

To test:

1. Login to an account with sites added
2. Once you're logged in tap on the My Site tab
3. Tap on the down arrow to open the site switcher
4. Tap the + button
5. Verify you see: 🔵 Tracked: add_site_alert_displayed <source: my_site>
6. Tap on the 'Create a WP.com Site' button
7. Verify you see 🔵 Tracked: enhanced_site_creation_accessed <source: my_sites>
8. Exit out of the flow
9. Logout, and sign back into your account
10. Once you reach the new Login Epilogue to select a primary site, tap the 'Create Site' button at the bottom
11. 🔵 Tracked: enhanced_site_creation_accessed <source: login_epilogue>
12. Finish the flow, and logout again
13. Create a new account
14. Once you get to the post sign up epilogue view, tap the 'Create a WP.com site' button
15. Verify you see 🔵 Tracked: enhanced_site_creation_accessed <source: signup_epilogue>
16. Exit out of the site creation flow
17. Tap on the My Site view
18. Tap on the 'Add Site' button
19. Verify you see: 🔵 Tracked: add_site_alert_displayed <source: my_site_no_sites>

Regression Notes

1. Potential unintended areas of impact
   Objective-C null values being passed to nonnull Swift function.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Made the swift function nullable and added a check ?? check.

3. What automated tests I added (or what prevented me from doing so)
   None.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[momo-ozawa]

Works as described!