Add unique constraint to `software_licenses.name` column.

The following discussion from !19598 (merged) should be addressed:

• @mbobin started a discussion: (+4 comments)

  safe_find_or_create_by! requires a unique index on name to work and it seems that we don't have it.

Additional information:

safe_find_or_create_by is used to create a new software license record using the name as the unique identifier for the record. However, the software_licenses.name column does not have a unique constraint.

• Introduced in 68a087e7
• Moved in ae997cdf

Implementation Plan

Backend - Done @xlgmokha

• Check if we have any name collisions in any of our environments.
• Check to see if it is safe to add a unique constraint to the software_licenses.name column without disruption.

Documentation - person

• User Documentation (is this needed?)