Substitue #to_s for Dashboard#title_attribute

[c-lliope]

title_attribute was being used to display a link to each resource
throughout the dashboards.

It was acting as a kind of unique identifier, but was restrictive in
several ways:

• The title_attribute had to correspond with a method on the model.
  It couldn't be computed from several methods.
• Only the current dashboard could look up the correct method for
  displaying a resource. This causes problems for relationships like the
  BelongsToAdapter and HasManyAdapter, which need to know how to display
  other objects.

Moving to #to_s is good in several ways:

• It makes representing an object super easy, with a single consistent
  method to call on each object.
• It encourages developers to write good #to_s methods on each of
  their objects.

And bad in several ways:

• If #to_s is not defined on a model, it defaults to the object's ID.
  This is not tied in any way to the database, so an object's identifier
  wouldn't be consistent throughout the dashboard.
• If the user hasn't defined #to_s on a model, it doesn't noisily fail
  by default. If we wanted to noisily fail, we'd have to check against
  the default #to_s string and raise an error.

To fix the bad effects, there's room for a separate gem that generates
better default #to_s methods for objects, by trying for attributes
like name or title, and defaulting to id if those are not present.

[jayroh]

I like this approach - super clean! 👍

[jayroh]

lgtm boss!