Preload certain public digest files for easy exploration

[alyssadai]

Goal is to provide a no-upload-option needed for regularly accessed digest files.

• Start with QPN, PPMI to come

• [x] Add UI element per input modality (imaging/phenotypic) that allows selection of a specific dataset's file

  • https://dash-bootstrap-components.opensource.faculty.ai/docs/components/dropdown_menu/
  • [x] use pattern-matching ID for callbacks

• [x] Do not trigger dataset name modal pop-up when digest file loaded instead of uploaded + pre-fill dataset name

• [x] Set repo paths for available dataset digest files

• [x] Turn example input file button into link in navbar

• [x] Separate upload buttons from dataset buttons with headings

• [x] Update upload prompt text

• [x] Check if tests need updating