Make the scrollbar always visible in lookback dropdown

[Parship999]

Which problem is this PR solving?

• Fixes Custom Time Range option is hard to spot #469

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: npm run lint and npm run test

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.13%. Comparing base (1c6fe9f) to head (f37c4bb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3048   +/-   ##
=======================================
  Coverage   98.13%   98.13%           
=======================================
  Files         257      257           
  Lines        8004     8004           
  Branches     2014     2014           
=======================================
  Hits         7855     7855           
  Misses        149      149           

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[Parship999]

here ::-webkit-scrollbar rules override the default scrollbar entirely

[yurishkuro]

Thanks!