refactor seeding files

In anticipation of future load testing where we will have to test thousands or hundreds of thousands of data items, I think it is useful to factor out the seeding for different models. It is also helpful when testing--for example I was working on adding the galleries seed data, but I didn't want to recreate the project/comments data each time I ran seeds.rb. Therefore I just commented out the other `require` lines in `seeds.rb` so I could just focus on galleries. 

We might have to change naming conventions over time, but I think this is a good start