don't write raw yaml

[michaelglass]

migrated from https://github.com/NoRedInk/NoRedInk/issues/11899

We should use the yaml library to export yaml instead of writing raw yaml. Writing raw yaml makes the code susceptible to a bunch of types of bugs (Escaping yaml primitives, etc) that using the library saves us from.

[BrianHicks]

This just bit us with a next-line (\u0085) character! So this bug is no longer hypothetical; even if it took a bit over four years to surface. Info about this character: https://www.unicode.org/standard/reports/tr13/tr13-5.html#Definitions