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 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.
Not doing so has a few gotchas when working with relations, particularly
with records which are not stored in the database.
I'm excluding the related content file because it's got a very peculiar
relationship with itself: the `has_one :opposite_related_content` has no
inverse; the relation itself is its inverse. It's a false positive since
the inverse condition is true:
```
content.opposite_related_content.opposite_related_content.object_id ==
content.object_id
```
This code was introduced in commit 722a431b, but it stopped being used
in commit 7657a0e0.
The spec was failing sometimes because the method didn't order the
records, but the spec checked the records returned in a certain order.
Furthermore, the method `page` generated a potential conflict with
kaminari's `page method.
The condition `params[:page_id] != 0` didn't work properly when editing
the homepage because in that case the parameter was `nil`, and the line
`SiteCustomization::Page.find(@card.site_customization_page_id)` raised
an exception because it couldn't find a page with a `nil` ID.
Fixing the issue while maintaining the check against `0` lead to complex
code, and so allowing `nil` in the database and assuming cards with no
`site_customization_page_id` belonged in the homepage seemed to be the
easiest solution.
