style(react-app): add default monospace font to font-family

[robscriva]

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable)

Description

This fixes style inconsistency where Roboto Mono isn't installed on the end-users machine. In this instance, it defaults to a serif font. It is defined properly in other parts of the React App codebase

bad

good

[github-actions]

Unit Test Results

     37 files  ±0       37 suites  ±0   28m 34s ⏱️ - 1m 33s
   591 tests ±0     539 ✔️ ±0  52 💤 ±0  0 ❌ ±0 
1 330 runs  ±0  1 262 ✔️ ±0  68 💤 ±0  0 ❌ ±0 

Results for commit a735fd1. ± Comparison against base commit a36fefa.

[shirshanka]

LGTM! Thanks for noticing and sending a fix @robscriva :)