feat: Models should download to a user-defined Directory

[dan-menlo]

Problem

• We currently download models to an app directory (e.g. Application Support for Mac)
• Problems
  • User cannot add/delete or copy/paste their own models in
  • Very dependent on our UI, which will take 1-2 months to stabilize
• We should consider downloading models to a user-defined directory in ~ or /documents
  • User can import their own models
  • We would have to develop a simple feature to scan the folder, and populate the DB
  • Alternatively show the model w/o metadata, and say that it was picked up from the folder

[freelerobot]

Dupe of #359

[freelerobot]

wont do. see ADR 5