Add user preference for monospace font

Issue

Our new monospace font has some usability concerns (check feedback for more details)

Proposal

Add a user preference to choose your preferred monospace font

Questions

1. Should this be site-wide or just in files?

🗂️ Just in files

• We could group all file preferences into a "Code" section or something. This could include:
  • Syntax theme
  • Diff colors
  • Monospace font
  • Single file code review
  • Whitespace changes
• This would mean we would potentially have two different monospace fonts across the product (in addition to GitLab Sans)
• We use monospace for text fields, which won't improve the usability problems reported by users

🌎 Across entire product

• One single monospace font across the product (in addition to GitLab sans)
• This would improve usability in text fields when reading and writing large amounts of text (for example, this issue description)
• We use monospace in small chunks, interspersed throughout the product (eg branch names and commits). If the user preference updates all instances, we'd need to make sure it looks uniform and polished site-wide

2. How would this preference look?

• Should we have a selection to choose from? Maybe 4 or 5 options?
• Maybe we could simplify by just choosing between JetBrains mono and our previous default System fonts