[ux] Add inspired favicon

[StephenHulme]

I had too many grey globes in my tab bar...

Changes proposed in this pull request

• Adds a Gatekeeper favicon inspired by @KatyTaylor
• Colours from Bootstrap 3 to match the Gatekeeper theme

Instructions for Reviewers

[All PRs] - Confirm PR template filled
[Feature Branches] - Review code
[Production Merges to main]
    - Check story numbers included
    - Check for debug code
    - Check version

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.24%. Comparing base (eba77c2) to head (a966639).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #669   +/-   ##
========================================
  Coverage    80.24%   80.24%           
========================================
  Files           74       74           
  Lines         1139     1139           
========================================
  Hits           914      914           
  Misses         225      225           

Flag	Coverage Δ
pull_request	80.24% <ø> (ø)
push	80.24% <ø> (ø)

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