General improvements and optimizations

[BobbyESP]

What has changed?

To start clarify that almost non part of the app has been afected itself, just the code has been improved and refactored to achieve the Kotlin code standarts.

• Deleted all the Ktor HttpEngine instances and left just one common for all (the same with the Json parser)
• Moved all DTOs to the new domain package and deleted some unnecessary.
• Made the GitHub API class an object
• Made the "Pure Black" switch perform in real time and not when restarting the app

Hope you like these changes!

[BobbyESP]

Have to fix this in plurals (fail of mine)

[BobbyESP]

@BobbyESP Fixed kek