Add support for themes to be stored in object storage

[LucidDan]

Currently, themes are loaded into the Docker image - uploading themes will fail because we have no persistent storage for them.

It'd be really cool if we could upload themes, have them stored in object storage (e.g. S3 etc), and lazy-load the themes from storage when they are activated.

[LucidDan]

Instead of lazy-loading when activated, I think the way to do this might be to simply have the startup script that I've already added do some additional magic to download and unzip the themes into the non-persistent ghost content directory.

Obviously we could also just require a persistent volume to be used, but my objective here is to avoid keeping any state on the Ghost node at all, so that it is really easy to run Ghost docker or kubernetes containers which can be migrated/deleted/re-created super-easily.