Factor out a helper for calculating next tile sort order

[jacobtylerwalls]

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Description of Change

This allows access to the calculation for tile sort order without having to call Tile.save(), e.g. if planning to call TileModel.objects.create(**data) instead.

Checklist

• I targeted one of these branches:
  • dev/8.0.x (under development): features, bugfixes not covered below
  • dev/7.6.x (main support): regressions, crashing bugs, security issues, major bugs in new features
  • dev/6.2.x (extended support): major security issues, data loss issues
• I added a changelog in arches/releases
• I submitted a PR to arches-docs (if appropriate)
• Unit tests pass locally with my changes
• I added tests that prove my fix is effective or that my feature works
• My test fails on the target branch

Ticket Background

• Sponsored by: Getty Conservation Institute
• Designed by: @jacobtylerwalls

[jacobtylerwalls]

I no longer plan to suggest the change in that TODO, so just rolling in the fix now.

[njkim]

lgtm