So now we'll be able to add them to other sections.
We're also adding a `dependent: :destroy` relation to models having
cards since it doesn't make sense to have cards around when their page
has been destroyed.
We use a different logic to load the card depending on the controller
we're using, and then share the rest of the code. This way we simplify
the code a bit, since we don't have to check for the page_id parameter.
We didn't add any validation rules to the card model. At the very least,
the title should be mandatory.
The fact that the label field is marked as optional in the form but the
other fields are not probably means description and link should be
mandatory as well. However, since there might be institutions using
cards with descriptions but no link or cards with links but no
description, so we're keeping these fields optional for compatibility
reasons. We might change our minds in the future, though.
We were repeating the same code over and over (with a few variants) to
setup tests which require an administrator. We can use a tag and
simplify the code.
