Refine string representation fo `ActiveRecord::Relation`

[raghubetina]

Right now, something like Photo.all will display something like

=> Array containing 323 Photo records

This patch proposes slightly different copy,

=> Photo::ActiveRecord_Relation (array with 323 Photo instances inside)

I suggest we do this along with splitting up the ActiveRecord Chapter into multiple chapters:

• Creating and modifying database tables
• Creating a single row (An ActiveRecord instance)
• Updating a single row
• Deleting a single row
• Retrieving a set of rows (An ActiveRecord::Relation)

And then we keep using the term "relation" heavily, rather than "array" or "collection", and demonstrate doing .class often after .all or .where or .order.

Thoughts?

[jelaniwoods]

LGTM 🚀 I intend to merge these changes in today.