Enable large headers / white nav bar feature flag

[frosty]

Refs #15750. This PR enables the large headers / white navigation bars feature flag for all users, for 17.0.

To test

There shouldn't be much to rest here, as the feature itself has been tested during implementation through other PRS.

• Build and run. Ensure you see white navigation bars, serif fonts in the navigation bars, and large header text for My Site, Reader, and Notifications.

PR submission checklist:

• I have considered adding unit tests where possible.
• 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 test the changes on this Pull Request by downloading it from AppCenter here with build number: 44306. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

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

[guarani]

Tested and working OK on an iPad, but on an iPhone 11 I couldn't get the site picker modal to show up after I'd opened the editor (both when running locally and when running the App Center build). It seems like switching tabs "resets" this though and allows me to open the site picker modal again.

Approving now since it seems like others haven't ran into these issues and we need to merge this for the 17.0 release cut.

[frosty]

Thanks @guarani! Going to go ahead and merge for now, as @Gio2018 was also unable to replicate, but we'll keep and eye on these issues during testing!