refactored db:seed for scalability, added another deck, altered migration, centered elements of progress bar

I have updated and refactored the seeds.rb db seeder to more easily accept new decks to be added to the db. I have included a GRE vocab deck. It should now be easier to add new decks. This could be refactored even further to accept merely a list of deck names, instructions and paths per deck.

The new deck GRE vocab deck I added broke the migration since it had questions longer than 255 characters. I changed the relevant field (cards.question) to t.text rather than t.string.

I vertically and horizontally centered the percentage sign in the progress bar by adding another div within the div and slightly altering the JS.