Reorganize app/ directory layout

[katyasoup]

PULL REQUEST

Summary

overview of new layout:
├── src
│   ├── app
│   │   ├── (pages) - note: parentheses are used to exclude the directory name from the url path
│   │   ├── api/ - import path updates only
│   │   ├── assets/ - not changed
│   │   ├── backend/ - import path updates only
│   │   ├── shared/ - see below
│   │   ├── tests/ - import path updates only
│   │   ├── ui/ - see below
│   │   ├── utils/ - import path updates only
│   │   ├── layout.tsx
│   │   ├── page.tsx
* also includes icon files at app/ level 


next-level contents:
├── src
│   ├── app
│   │   ├── (pages) - components or groups of components that are unique to a single page view/not re-usable
│   │   │   ├── fhir-servers/
│   │   │   ├── query
│   │   │   ├── queryBuilding/
│   │   │   ├── signin/
│   │   │   └── userManagement/
│   │   ├── shared/ - files that are used by both the front and back ends
│   │   │   ├── constants.ts
│   │   │   ├── CustomQuery.ts
│   │   │   ├── database-service.ts
│   │   │   ├── DataProvider.tsx
│   │   │   ├── fhir-servers.ts
│   │   │   ├── format-service.tsx
│   │   │   ├── query-service.ts
│   │   │   ├── seedSqlStructs.ts
│   │   │   └── utils.ts
│   │   ├── ui
│   │   │   ├── components/ - complex groups of elements that are re-usable between pages
│   │   │   ├── designSystem/ - basic elements and simple groups of those elements
│   │   │   └── styles/ - base styles and settings / configs for scss variables, etc. 
│   │   │    * designSystem, component, and page styles should live in their respective directories

Related Issue

Fixes #180 #316 #206

Additional Information

Anything else the review team should know?

Checklist

• Descriptive Pull Request title
• Link to relevant issues
• Provide necessary context for design reviewers
• Update documentation

[johanna-skylight]

LGTM! with my limited knowledge I tested in local and all look good.