Using "data-target", it is possible to set a value to a target element
that can enable it or disable it. Using "data-context" lets us perform
the opposite action on a different target.
Before, only the #external_wiki_box target was used, which was enabled
or disabled depending on whether the user has chosen to use the internal
wiki or the external wiki. If the user chooses to use the internal wiki,
they will disable the box that lets them enter a link pointing to an
external wiki, and vice versa. Although it is not possible to use, say,
boolean operations, we can introduce a target that is
called #globally_writeable_checkbox that gets enabled when
the #external_wiki_box box is disabled, and vice versa.
This makes the box's behavior more consistent with the behavior in the
"Issues" section. To keep things consistent with that section, a new
property was assigned to the "globally_writeable_checkbox" that makes
the box go a bit further in (`tw-pl-4`).
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