Maintenance and refactoring of current code base

[ScriptyChris]

A group of actions needed to improve app further development, building and testing:

• configure CI/CD [partially done in Prepared build process for CI/CD #26]
• add unit tests for backend
• refactor backend code to use TypeScript
• unify HTTP statuses being send from backend into enum and make handling HTTP errors (including frontend) more consistent
  • backend part done in Refactor API error handling to be more consistent #45
  • frontend part
• add E2E tests for frontend [started in Introduce E2E tests #49]
• refactor frontend code to use TypeScript [started in Started frontend codebase migration to TypeScript #48]
• use Docker to unify and facilitate way of distributing the app [partially done in Prepared build process for CI/CD #26]
• add a initial config endpoint to let frontend fetch essential constants (like categories tree and technical specs nesting level or user account types or maximum product rating value) and cache it until config update is done on backend
• add code documentation (with i.e. JSDoc) and update README.md with it's generated markdown output (via i.e. jsdoc-to-markdown) [Introduce API documentation #58]
• refactor (frontend) routing to contextually nest/group views [Refactored frontend routing and extended middleware response wrapper #47]
• refactor middleware-response-wrapper.ts module to wrap whole (chained) calls to res.status(..), res.json(..), res.sendStatus(..) etc. to have more robust type checking between HTTP status code and response data kind (i.e. 4xx HTTP codes may only be used with response containingerror data) [Refactored frontend routing and extended middleware response wrapper #47]
• remove models-index.ts (re-exporting) module and instead import each Schema/Model stuff by individual modules.
  [update] this module was renamed and refactored in Refactored database models and API #54