Clean and improve seeds

[mkasztelnik]

This PR includes:

• csv files moved into db/seeds dir
• activerecord-import gem used for bulk insert.

Before bulk insert:

bundle exec rake db:seed  24,74s user 1,35s system 40% cpu 1:04,59 total

After bulk insert:

bundle exec rake db:seed  14,84s user 0,57s system 92% cpu 16,715 total