Rename Subscribers notification details header

[justtwago]

Fixes https://github.com/Automattic/wordpress-mobile/issues/61

It's the final part of the Subscribers Details screen redesign. The problem is that every notification response contains a "title" property that is applied to a notification details screen's title. I made this little workaround to rename Followers to Subscribers without touching the backend.

To test:

• [ ] Prepare a notification about new subscribers
• [ ] Go to the screen and compare it with the designs
• [ ] Make sure you can see the redesigned name of the feature "Followers" -> "Subscribers."

Regression Notes

1. Potential unintended areas of impact Notifications details

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

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

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.

Testing checklist:

• [ ] WordPress.com sites and self-hosted Jetpack sites.
• [ ] Portrait and landscape orientations.
• [ ] Light and dark modes.
• [ ] Fonts: Larger, smaller and bold text.
• [ ] High contrast.
• [ ] VoiceOver.
• [ ] Languages with large words or with letters/accents not frequently used in English.
• [ ] Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• [ ] iPhone and iPad.
• [ ] Multi-tasking: Split view and Slide over. (iPad)