70513102 performance of contents index pages

https://www.pivotaltracker.com/story/show/70513102

- ContentPlans index page tweaks:
  - added eager load for tags (act_as_taggable_on's tag_list ignores .includes(:tags))
  - corrected fetching of contents sum of sizes ( contents.sum(:size) ignores .includes(:contents))

- Contents index page tweaks:
  - set fetching of tags and users via eager loading