[Service - ApiDocs relationship] Adapt Liquid Drops to it

[Martouta]

Depends on #62 in which ActiveDocs and Service have a relationship.

fixes https://github.com/3scale/porta/issues/61 and belongs to THREESCALE-1340

1. Service drop -> api_specs
   http://provider-admin.example.com.lvh.me:3000/p/admin/liquid_docs#service-drop
   
2. ApiSpecs drop -> service
   http://provider-admin.example.com.lvh.me:3000/p/admin/liquid_docs#apispec-drop
   
3. Improve the already existing ProviderDrop#api_specs documentation
   http://provider-admin.example.com.lvh.me:3000/p/admin/liquid_docs#provider-drop
   
   Removing the access to deleted api_specs from provider for the service is already done in [Service - ApiDocs relationship] Create relationship and adapt endpoints for Create and Update (UI & API) #62

[Martouta]

I didn't know if in this case I should return nil or an empty service drop, but I saw that for other cases we are currently returning nil so I kept it the same way. For example in:

porta/lib/developer_portal/lib/liquid/drops/service_contract.rb

Lines 29 to 33 in 029cba3

	def service
	if s = find_service
	Drops::Service.new(s)
	end
	end

[guicassolato]

If it's nil then it can be less verbose

return unless service = @spec.service

[guicassolato]

Can we backquote nil?

[Martouta]

No idea 😱 This is autogenerated, but I will try to accomplish that 😄 Maybe it is as easy as typing backquoted the original text 😊 I will try 😄

[Martouta]

You were right, it was that easy and it looks better 👍 💪