Improvements: Run web only if enabled + Rename svc-beets + Add s6 overlay for drop2beets

[dtrunk90]

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• [x] I have read the contributing guideline and understand that I have made the correct modifications

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Description:

1. Run web only if enabled: This fixes #141
2. Rename svc-beets: In order to reflect what this s6 overlay does it's better to name it beets-web since the only purpose of this overlay is to run the web plugin as a longterm service
3. Add s6 overlay for drop2beets: This is not enabled by default but the required s6 overlay to be able to enable it was added here

Benefits of this PR and context:

It fixes #141 and all users would benefit from the other commits

How Has This Been Tested?

I've used the following docker-compose.yml to run and test my changes:

services:
  beets:
    build: .
    container_name: beets
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=Europe/Berlin
      - DOCKER_MODS=linuxserver/mods:universal-package-install
      - INSTALL_PIP_PACKAGES=drop2beets
    volumes:
      - ./config:/config
      - ./dropbox:/dropbox
      - ./logs:/logs
      - ./music:/music
    restart: unless-stopped

• I've checked w/o plugin web if the error message error: unknown command 'web' dissappeared.
• I've checked w/ plugin web if the web service is up and running
• I've checked w/ plugin drop2beets if the service is up and running

Source / References:

https://github.com/linuxserver/docker-beets/issues/141 https://github.com/linuxserver/docker-beets/pull/138