Rewards: Add "Hide Rewards Icon" setting to new Display settings section

[kylehickinson]

Fixes brave/brave-rewards-ios#203

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

• Prefix: Enable rewards by removing the NO_REWARDS build flag
• Verify that rewards is disabled/wallet not created
• Open settings, verify that new display section looks correct and settings within it work as usual
• Turn on Hide Brave Rewards Icon and close settings, verify that the icon is no longer in the toolbar
• Verify that icon stays hidden through app launches
• Turn off Hide Brave Rewards Icon and enable Brave Rewards in the panel
• Turn on Hide Brave Rewards Icon again and verify that the icon is still visible in the toolbar since BR is enabled

Screenshots:

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • release-notes/(include|exclude)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue is assigned to a milestone (should happen at merge time).

[kylehickinson]

This needs a transfix push btw