Add go to definition support for through: and class_name: association options

[sucicfilip]

through: option

When clicking on the through: value, it now navigates to the association definition in the same file:

class Organization < ActiveRecord::Base
  has_many :memberships
  has_many :users, through: :memberships
  #                         ^^^^^^^^^^^^
  #                         go to definition -> has_many :memberships (line above)
end

This implements the behaviour mentioned in the Rafael's comment below - navigating to the association declaration rather than the model, since through: references another association, not a model directly.

class_name: option

When clicking on the class_name: value, it navigates to that class using the index (no server call needed):

class User < ActiveRecord::Base
  has_one :location, class_name: "Country"
  #                              ^^^^^^^^^
  #                              go to definition -> Country class
end

Additionally, when clicking on the association name that has a class_name: specified, it uses the index directly instead of making a server call:

class User < ActiveRecord::Base
  has_one :location, class_name: "Country"
  #       ^^^^^^^^^
  #       go to definition -> Country class (via index, no server call)
end

This provides a small performance optimization since we can resolve the class name directly from the code without asking Rails for reflection data.

---

Closes #676

[rafaelfranca]

I think the right thing to do is to go to has_many :memberships definition if you are trying to go to the definition of : memberships.

[sucicfilip]

We can save a server call here.

If an association is defined so that the class_name is explicitly stated, we can just use that instead of making a server call.