Add "Show age indicator" toggle in blame preferences

Context

This is a follow-up issue from #561788 (Adjust the layout of the gutter on the inline blame mode on the blob page).

Proposal

Add a toggle in the blame preferences to show/hide the age indicator legend ("Newer 🟦 🟦 🟦 Older").

Per the design proposal:

Toggles the "Newer 🟦 🟦 🟦 Older" age indicator legend on/off. By default this is off, as it's redundant with the actual text and the legend is learnable.

Design Reference

See the Figma designs linked in #561788:

Without legend With legend
2.1 blame-oneline_ no key.jpg 2.2 blame-oneline_ show key.jpg

Acceptance Criteria

  • Add "Show age indicator" toggle to blame preferences
  • Toggle is OFF by default
  • Age indicator legend appears/disappears based on toggle state
  • User preference is persisted