Correct parent tag on mainstream category

The slugs for a set of visa-related browse pages were updated in
panopticon (https://github.com/alphagov/panopticon/commit/ce57246d1c34eac4b37eee7d478535cdcdcc9fca)
but the whitehall mainstream categories were not updated accordingly,
specifically the "Endorsing, sponsoring and supporting visa applicants"
category that has the parent tag of
"visas-immigration/sponsoring-workers-students". This has caused any
detailed guides tagged to this category to generate broken links, for
example the link to the mainstream category in the meta data for this
guide (https://www.gov.uk/apply-for-a-tier-4-sponsor-licence) is broken.
This data migration updates the category's parent_tag to match the slug
of the section in panopticon.
Merge pull request #1583 from alphagov/fix-broken-mainstream-category

Correct parent tag on mainstream category