And remove snapshots (we don't need it anymore)

- Implement new API
- Migrate the LineDecorator
- ensure no change on the "lines/index" view

- Add `activate` and `deactivate` actions in `LinesController`, as well
as corresponding routes
- Add `activate!` and `deactivate!` methods in `Chouette::Line`, as well
as `activated?`
- Add `activate?` and `deactivate?` permissions in `LinePolicy`
- Add corresponding `action_links`in the Decorator
- Create helper for these actions
- Add an optional `'extra_class` to the Links
- Update styles for ".delete-action" to handle the case where there are
several
- Add I18n keys accordingly

The tests failed because we weren't using a decorated `Chouette::Line`
object, but the views now expect decorated lines in order to be able to
call the `#action_links` method.

Here are our failures:

    5) /lines/show should render h1 with the line name
       Failure/Error: - @line.action_links.each do |link|

       ActionView::Template::Error:
         undefined method `action_links' for #<Chouette::Line:0x007fc8cdf0bb88>
       # ./app/views/lines/show.html.slim:10:in `block in _app_views_lines_show_html_slim___3813514632397395671_70250211127360'
       # ./app/helpers/newapplication_helper.rb:246:in `block (2 levels) in pageheader'
       # ./app/helpers/newapplication_helper.rb:244:in `block in pageheader'
       # ./app/helpers/newapplication_helper.rb:243:in `pageheader'
       # ./app/views/lines/show.html.slim:2:in `_app_views_lines_show_html_slim___3813514632397395671_70250211127360'
       # ./spec/views/lines/show.html.erb_spec.rb:16:in `block (2 levels) in <top (required)>'
       # -e:1:in `<main>'
       # ------------------
       # --- Caused by: ---
       # NoMethodError:
       #   undefined method `action_links' for #<Chouette::Line:0x007fc8cdf0bb88>
       #   ./app/views/lines/show.html.slim:10:in `block in _app_views_lines_show_html_slim___3813514632397395671_70250211127360'

    6) /lines/show should render a link to remove the line
       Failure/Error: - @line.action_links.each do |link|

       ActionView::Template::Error:
         undefined method `action_links' for #<Chouette::Line:0x007fc8cfdaa710>
       # ./app/views/lines/show.html.slim:10:in `block in _app_views_lines_show_html_slim___3813514632397395671_70250211127360'
       # ./app/helpers/newapplication_helper.rb:246:in `block (2 levels) in pageheader'
       # ./app/helpers/newapplication_helper.rb:244:in `block in pageheader'
       # ./app/helpers/newapplication_helper.rb:243:in `pageheader'
       # ./app/views/lines/show.html.slim:2:in `_app_views_lines_show_html_slim___3813514632397395671_70250211127360'
       # ./spec/views/lines/show.html.erb_spec.rb:31:in `block (2 levels) in <top (required)>'
       # -e:1:in `<main>'
       # ------------------
       # --- Caused by: ---
       # NoMethodError:
       #   undefined method `action_links' for #<Chouette::Line:0x007fc8cfdaa710>
       #   ./app/views/lines/show.html.slim:10:in `block in _app_views_lines_show_html_slim___3813514632397395671_70250211127360'

Update the test to decorate the lines used in them.

Turn `current_organisation` into a `context` field on `LineDecorator`
because otherwise it's a global variable that comes from wherever that's
required in `LineDecorator`. And we'd have to mock
`current_organisation` on `line` in the test, which I wasn't keen on.

Refs #3479