use more compatible flexbox align-items: flex-start value
Created by: sqs
Fixes CSS compatibility warning: start value has mixed support, consider using flex-start instead
(from Webpack).
See https://stackoverflow.com/questions/50919447/flexbox-flex-start-self-start-and-start-whats-the-difference for more information. The 2 values are different (so this can't be automatically fixed in PostCSS), but for our purposes they are equivalent, so it's better to use the more compatible one.