Use a modern font stack inspired by Bootstrap 4

[Calinou]

This makes the frontend use a better-looking, more consistent font stack.

Preview

Note: The font used depends on the OS (as before). Screenshots below were taken on Fedora 31 with Microsoft fonts installed.

Before

After

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @smarnach (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[Calinou]

Is there anything I can do to help this get merged? I can notice some CI failures below, but I'm not sure if they're related to my PR.

[JohnTitor]

@Calinou IIRC it's a known issue and should be fixed in latest nightly, so unrelated. If you want to re-trigger CI, you can rebase or close/re-open the PR.

[Calinou]

Can I update the Percy screenshots on my own to make the visual regression tests pass, or does someone else have to do it?

[jtgeibel]

@Calinou I've approved the visual changes in Percy. Lets go ahead and merge, sorry for the delay.

@bors r+

[bors]

📌 Commit aaf62aa has been approved by jtgeibel

[bors]

⌛ Testing commit aaf62aa with merge 527c8dd...

[bors]

☀️ Test successful - checks-travis
Approved by: jtgeibel
Pushing 527c8dd to master...