Referentials#show has a button "Purger" that isn't a link. Instead, it's
a `<button>` element that opens a modal.

Previously, we only knew how to put `Link`s in the `#action_links`, and
only these could then be rendered in the header bar as buttons.

This change allows us to accept non-Link objects. Since the button is
rather simple, I decided to create a simple `HTMLElement` class to
represent it and output it in the resulting HTML. The class uses the
`#content_tag` `ActionView` helper to render the final HTML.

Update "referentials/show.html.slim" to output the button correctly in
addition to the normal links.

Update `TableBuilderHelper#build_links` to not add anything that's not a
`Link` to the gear menu. This allows us to display the "Purger" button
on the Referentials#show page but not in the table on Workbenches#show.

Refs #3479

The only difference between :name and :content was that :name contained
a string and :content contained HTML. But they're really the same thing.
Thus it makes much more sense to combine those into a single property
that means both, and handles content regardless of type.

Refs #3479

Previously we weren't correctly outputting the proper HTML to get the
:delete link rendering correctly with the icon and text. It should show:

    [TRASH CAN ICON] Supprimer

but instead was showing the href.

To get this working, I added a `@content` attribute to `Link`. This
allows us to optionally specify non-text content to display in the link.

We then have delete-link-specific handling when rendering the gear
menu's HTML. In that particular case, the `<li>` needs to have a
'delete-action' class in order for the styling to work properly.

Created a new `destroy_link` helper method to allow us to reuse the
destroy link content in the header bar.

TODO: Collect Link#content and Link#name into the same property.
TODO: Rename `#destroy_link` to `#destroy_link_content`

Refs #3479

Don't set links as `method: :get` by default. This shows up as:

    link_to _, _, method: :get

If we set the method to `nil` instead, we don't get a
`data-method="get"` attribute in our HTML output. Instead, we get no
`data-method` attribute, which is a lot better and cleaner.

I had originally used `:get` as a default because I wanted to generalise
links and certain ones we create need `:delete` or `:put` methods. I
figured, since we're always going to be passing `method:` to `link_to`,
we should have a sane default for that option. However, using `nil` is
even better as a default because then we don't get an extra attribute in
our HTML at all.

Refs #3479

Add a new `ReferentialDecorator` that we can use to define the links
that appear as buttons in the header of the `/referentials/:id` page.

The idea is that these links will take the place of the existing view
code and in the view, we'll instead loop over the links provided by the
decorator and render them as buttons.

I decided to do it this way because there's a requirement that the links
in the gear menu in tables show the same links that appear in the header
of the page. So the Workbenches#show page would show a table of
referentials, and their gear menu links should ostensibly be the same as
the header links in the Referentials#show page.

My goal was to abstract links, and try to separate them from the
presentation layer. Still having trouble with this though. I created a
new `Link` class to represent a link. We can then use this in the
template to create a `link_to`. It becomes messy, though, when we want
to put other elements inside a certain link, as evidenced by the
`:delete` link. Don't like how that's done, but still working out the
best approach to make it cleaner.

Refs #3479