Page Settings: set display titles to "Page Settings" if a page is being editing.

[ScoutHarris]

Fixes #12374

When editing a page, change settings titles to Page Settings instead of Post Settings.

To test:

• Edit a Page.
  • Select ... on the nav bar.
  • Verify the settings option is Page Settings.
• Switch to classic editor.
  • Select ... on the nav bar.
  • Verify the settings option is Page Settings.
• Select Page Settings.
  • Verify the view title is Page Settings.

Gutenberg menu	Classic menu	Settings view

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 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.

[mindgraffiti]

[ScoutHarris]

Thanks @mindgraffiti !