Restore missing features: haproxy and file manager

[kbumsik]

Hi, Since I have used this project on a daily basics (mainly for Airplay though) I noticed a few things:

• There is no convenient way to upload files to the persistent storage for Mopidy. File manager was added in #25 for this, but it was excluded in the docker-composer.yml since the project was migrated to multi-containers configuration.

• Although the Wiki states that I can access to the services using path like http://boombeastic_ip/**emoji/** , I cannot use those paths as haproxy is not enabled in docker-composer.yml as well.

This PR addresses those problems:

• Restore Filemanager and make Mopidy handle files better (update file changes).
• Restore haproxy.

It could be two separated PRs but the changes are rather small, so I decided to propose them in a single PR.

BTW, please consider to modify the permissions of the Wiki page so that users like me can fix some errors.

[curcuz]

Hey @kbumsik thanks a lot for taking care of this, I've been very busy and this slipped under my radar :)

[kbumsik]

@curcuz You must have been very busy, looking at the latest news from Balena :) Congrats on the Series B funding! It was my mistake to forget mentioning why I removed the websocket proxy. I removed it as expose: "1337" in the docker-compose.yml will do enough job.