Change existing migration to create Category

[JPrevost]

Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/TCO-125

How does this address that need:

• Creates a Category "Flagged" if one does not already exist before trying to update data based on this specific Category

Document any side effects to this change:

• This migration has already run and it is normally not a good practice to update migrations to prevent databases from not bein in synch with the migration. In this case, all this does is introduce the code that was run manually in environments where this was run so it is fine and expected that those environments will not run it again.

Developer

Accessibility

• ANDI or Wave has been run in accordance to our guide and
  all issues introduced by these changes have been resolved or opened
  as new issues (link to those issues in the Pull Request details above)
• There are no accessibility implications to this change

Documentation

• Project documentation has been updated, and yard output previewed
• No documentation changes are needed

ENV

• All new ENV is documented in README.
• All new ENV has been added to Heroku Pipeline, Staging and Prod.
• ENV has not changed.

Stakeholders

• Stakeholder approval has been confirmed
• Stakeholder approval is not needed

Dependencies and migrations

NO dependencies are updated

YES migrations are included

Reviewer

Code

• I have confirmed that the code works as intended.
• Any CodeClimate issues have been fixed or confirmed as
  added technical debt.

Documentation

• The commit message is clear and follows our guidelines
  (not just this pull request message).
• The documentation has been updated or is unnecessary.
• New dependencies are appropriate or there were no changes.

Testing

• There are appropriate tests covering any new functionality.
• No additional test coverage is required.

[matt-bernhardt]

This looks like copy-pasting that line from db/seeds.rb into the migration, right? That's what I've been doing manually, so this looks good.