[Guided Setup] Optimize default tree creation

[alesan99]

Fixes #7641

Speeds up default tree creation significantly.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests
• Add a reverse migration if a migration is present in the PR

Testing instructions

• Test with the setup page
  • Use a blank database
  • Pre-load all the trees you can and complete the setup
  • Make sure the trees were pre-loaded correctly.
• Test in the taxon tree viewer
  • Add a new taxon tree and choose to import a populated one.
  • Make sure the tree was created correctly. An easy way to check is to compare it with the same tree created before the speed ups.