Separate the app into an API and Web UI

[lasersPew]

Is your feature request related to a problem? Please describe.
Being a monolithic project is kind of a pain. For one, it doesn't allow scalability esp. in deployments like docker.

Describe the solution you'd like
I want this project to be MetaTube-API and MetaTube-Web

Describe alternatives you've considered
None

Additional context
This may also ease up the troubleshooting for server or client specific problems. It also make me and other maintainers do #94, #84, #2, etc. Next, things like multi-user support may easily be implemented. Additionally, it allows scalability (esp for power users of Docker and other tools). And lastly, this also allows integration for selfhosted music players to have access to one's library.