Note: Did not fix the missing workgroup in the calendar mailer

Previously, we weren't correctly adding the HTTP method to the links in
the header. This is because it's called `link_method` in the class. Put
this value in `method`/`data-method` when converting the link to HTML so
that the links work properly.

Discovered this when deactivating and activating stop areas on
http://stif-boiv.dev:3000/stop_area_referentials/1/stop_areas/139429 .

Refs #5586

This method was overriding classes that were added by `#add_class` in
addition to those added by `#class`. Obviously, the argument to
`#add_class` should be appended to the default.

That seemed like a pain to do, so instead I'm getting rid of the concept
of a default class. Instead, we're going to force all links to use the
template-defined styles for now. That goes in line with Luc's desire to
be more consistent in the links, as he stated yesterday.

Refs #5586

Instead of checking for an existing class and then setting the class,
provide a method to set a default class when one hasn't been defined.

This does effectively the same thing but moves the condition inside the
method.

Thanks Johan for the suggestion!

Refs #5586

Begin the process of converting the `ImportDecorator` to the new action
links interface.

For now, only the "New import" button on the top right of the index page
is converted. The rest of the links are commented out for now and will
be treated in a subsequent commit.

Add a new `#class?` method to `AF83::Decorator::Link` that tells callers
whether a CSS class has been defined on the link. This allows us to
determine whether we should force a class or use the one provided in
`_page_header.html.slim`.

We need a way to specify the class of the link in the header because the
"New import" button is supposed to be blue (`.btn-primary`), but the
default for primary action links is white.

Refs #5586