Introduce UserRepository abstraction layer and Introduce Mappers

[JasonChong96]

Problem

According to #156, the back-end directory structure beginning to look incoherent and is showing signs of unbridled growth. It is currently a good time for refactoring given the recent expansion of file upload feature.

This PR continues the process of addressing this by creating a UrlRepository which handles the data store of users.

With the introduction of UserRepository, multiple classes need to share usage of mapper functions which map persistence (sequelize) objects to data transfer objects.

Solution

Introduce UserRepository to further abstract the data store from other services.

Improvements:

• Mapper classes and interfaces have been introduced to map between persistence and dto's.
• Code coverage increased somehow

Tests

• Creating URL works
• Transferring ownership of URL works
• Editing of URL works
• Redirect works
• Toggling state to active/inactive works and is reflected accordingly
• Repeat all previous steps for files
• Logging in works
• Verifying OTP works

Additional notes
There are some consecutive lines such as these
persistenceToDto(urlType: UrlType): StorableUrl
persistenceToDto(urlType: UrlType | null): StorableUrl | null {
in the mappers. These are there to tell the typescript compiler to that the method returns a nullable value if and only if the input is nullable.

[liangyuanruo]

lgtm barring one comment