Refs #3851

Refs #3479

Get the header links from the decorator so we can reuse them instead of
defining them directly in the template.

Add an option to the `destroy_link_content` Rails helper method to allow
us to specify alternate translation keys, giving us different text in
the button label. Added this because the template in question uses a
different label than the others (usually it's `actions.destroy`).

Refs #3479

Add a few doc comments to the table builder to provide a little more
detailed explanation of the code.

TableBuilderHelper::URL:
* Describe dependency on `current_referential`

TableBuilderHelper:
* Add doc comments to the `collection` and `columns` arguments
* Add a description of the module
* Detail external global variable dependencies
* Provide an example, stolen from one of the tests

Refs #3479

We were using both the instance variable and the accessor on the same
line. Pick one and be consistent.

Refs #3479

Keep the helper code globally the same.

Changes:
* Remove `if action == :edit` since we never execute that branch. In
  fact, the only place we ever use the multiple selection toolbox in
  conjunction with the table builder is on Workbenches#show, and that
  page only uses a :delete action.
* Rename `dPath` to be clearer
* Add some whitespace to improve readability
* Rename `actitem` to `action_link` for better readability
* Connect the links and label with `#+` instead of `#concat`

Add the toolbox to "workbenches/show.html.slim" to get the page looking
like it was before I changed the table builder.

Needed to wrap the table and toolbox in a `<div class="select_table">`
in order for the JavaScript part of it to work (otherwise selection
wouldn't activate the link, and likely other stuff would be messed up
too).

Now that the table builder and the selection toolbox are separate, it
makes sense to put the "select_table" `<div>` in the template.

Refs #3479

Port the `select_toolbox` from `NewapplicationHelper`. For now the code
is the same. Just the name and method comment have changed.

Refs #3479

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

I added these comments when I was reading the code and trying to
understand it. Now I basically understand all of it, and we've ported
the code to `TableBuilderHelper`. These comments can be removed to
revert the file back to how it looked before I messed with it.

Refs #3479

Remove unnecessary arguments and document the ones we are using.

`current_referential` has been removed. Ultimately decided that it was
simpler to just rely on the existing method that's magically "imported"
instead of passing it directly. If you vote to pass it in as an arg,
though, I'm all for it.

Refs #3479