allow a theme to specify a font - #225

Created by: dakkar

What

With this, admins and users can specify a font in their themes. Any value that would work as the source argument to new FontFace can be used:

• local(Arial)
• url(https://fonts.gstatic.com/s/opensans/v36/memSYaGs126MiZpBA-UvWbX2vVnXBbObj2OVZyOOSr4dVJWUgsjZ0C4n.ttf) format('truetype'), local(Helvetica), local(Arial)

they all work.

Why

Do you want a specific dyslexic-friendly font? You can have it!

Do you want to make your instance's text unreadable as a prank to your users? Sure!

Additional info (optional)

I'll write a docs page for this.

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests