Switch from Paperclip to ActiveStorage

[CyanoKobalamyne]

Paperclip was deprecated in May 2018: https://thoughtbot.com/blog/closing-the-trombone. Rails ActiveStorage can be used instead, which also has an AWS S3 backend.

Example article that describes a large-scale migration: https://medium.com/sortlist-engineering/migrating-from-paperclip-to-activestorage-a-different-approach-4bffa2260e17.

[bkettle]

May be less relevant in light of #575. Seems like it would still eventually be good to move to ActiveStorage, but the immediate need is avoided for now.

[CyanoKobalamyne]

Yes, I think the benefit is a reduced dependency tree size (and hence reduced exposure to bugs). But there are likely many higher-priority issues...