Refactor images

[oskarrough]

This PR closes #161. It does so by

• removing our image model
• replacing channel.images (hasMany) with channel.image (string)

It requires a node.js script that takes the newest image on each channel and puts the id as channel.image. I already ran this on staging but did not push the script anywhere.

TO PUT IT LIVE

• deploy api to production (internet4000/radio4000-api-node-firebase@24cc610)
• merge this to master, then production
• run the script on the live db
• no longer need https://github.com/internet4000/radio4000-player/blob/master/src/utils/store.js#L53 --> Refactor to new image format radio4000-player#124

Related:

internet4000/radio4000-api-node-firebase#8
internet4000/radio4000-api-node-firebase#7
#248

[4www]

Let's take care, as now that this is merged to master, the next time we merge to production we will have conflicts with how the player handles images.