Explain why we need to skip and un-skip the callback.

Refs #6095

Since this method was run on `after_save` and in it
`RouteWayCostCalculator` calls `#update` on the given route, it caused
an infinite recursive call to the worker. To prevent this, wrap the
`#calculate!` call in methods that unset and reset the callback.

Refs #6095

Just running `Chouette::Route.find(id)` doesn't give us anything if we
don't have a `Referential` selected. In order to properly get the
correct route, pass the referential ID to the worker and switch to that
referential before trying to select the route.

Refs #6095

This new method will launch a worker that takes the route's `StopArea`s,
converts them to `WayCost`s, sends them to the TomTom API to calculate
distance and time costs, and saves those costs to the route's `costs`
field.

Refs #6095

Also added a *dumb* `WorkgroupExport` for testing purposes.

I was getting this error when I removed the `rescue nil`s:

    Failures:

      1) WorkbenchImportWorker foreign lines uploads the following entries: ["OFFRE_TRANSDEV_20170301122517", "OFFRE_TRANSDEV_20170301122519"]
         Failure/Error: eg_file.unlink

         NoMethodError:
           undefined method `unlink' for #<File:0x007fef1adbace8>
         # ./app/workers/workbench_import_worker.rb:81:in `upload_entry_group_stream'
         # ./app/workers/workbench_import_worker.rb:52:in `upload_entry_group'
         # ./app/services/zip_service.rb:61:in `<<'
         # ./app/services/zip_service.rb:61:in `finish_current_output'
         # ./app/services/zip_service.rb:35:in `add_entry'
         # ./app/services/zip_service.rb:27:in `block in _subdirs'
         # .../.gem/ruby/2.3.3/gems/rubyzip-1.2.1/lib/zip/entry_set.rb:38:in `block in each'
         # .../.gem/ruby/2.3.3/gems/rubyzip-1.2.1/lib/zip/entry_set.rb:37:in `each'
         # .../.gem/ruby/2.3.3/gems/rubyzip-1.2.1/lib/zip/entry_set.rb:37:in `each'
         # .../.gem/ruby/2.3.3/gems/rubyzip-1.2.1/lib/zip/central_directory.rb:182:in `each'
         # ./app/services/zip_service.rb:26:in `_subdirs'
         # .../.gem/ruby/2.3.3/gems/rubyzip-1.2.1/lib/zip/file.rb:133:in `open_buffer'
         # ./app/services/zip_service.rb:21:in `block in subdirs'
         # ./app/workers/workbench_import_worker.rb:39:in `each'
         # ./app/workers/workbench_import_worker.rb:39:in `each_with_index'
         # ./app/workers/workbench_import_worker.rb:39:in `upload'
         # ./app/workers/workbench_import_worker.rb:19:in `perform'
         # ./spec/workers/workbench_import_worker_spec.rb:68:in `block (4 levels) in <top (required)>'
         # ./spec/workers/workbench_import_worker_spec.rb:68:in `block (3 levels) in <top (required)>'
         # ./spec/workers/workbench_import_worker_spec.rb:13:in `instance_eval'
         # ./spec/workers/workbench_import_worker_spec.rb:13:in `block in expect_upload_with'
         # .../.gem/ruby/2.3.3/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:268:in `load'
         # .../.gem/ruby/2.3.3/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:268:in `block in load'
         # .../.gem/ruby/2.3.3/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:240:in `load_dependency'
         # .../.gem/ruby/2.3.3/gems/activesupport-4.2.8/lib/active_support/dependencies.rb:268:in `load'
         # .../.gem/ruby/2.3.3/gems/spring-commands-rspec-1.0.4/lib/spring/commands/rspec.rb:18:in `call'
         # -e:1:in `<main>'

Johan said that in his tests on his local machine, the temp files
weren't getting deleted. When I try running the specs, the temp files do
get deleted, so not sure what was going on. Figure it may be because of
the `rescue nil`s, so I decided to remove those in the hopes of always
ensuring we close and unlink the temp files.

The trouble with doing that, however, was that it caused the error and
thus test failures like the example above. I managed to fix the problem
by using `File.unlink` instead of the instance method.

Hopefully this works to ensure that the temp files are always deleted.

Refs #4315

If the directory doesn't exist, we should create it to ensure we don't
get errors trying to write to a location that doesn't exist. I had
removed this because I assumed the dir would be created during
deployment, but I guess I shouldn't assume. Also, we want to ensure the
dir gets created on dev machines without errors.

This line is updated from last time where we'd create
`Rails.root.join('tmp', 'imports')`. Now we use our `#temp_directory`,
which can be variable.

Refs #4315