Add paperclip back to the gemfile so legacy migrations work.

[seanmarcia]

I had a new contributor complain that running rails db:migrate didn't work and that our migrations were broken.

It turns out that since we used to have paperclip in the application and we had a migration that depended on it we need to put this back to let new folks be able to run the migrations.

This is obviously a short term fix. We'll need to eventually remove the gem and the fix the migrations but this immediately unbreaks things and lets new contributors run migrations as expected.

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