Implement creating new entries

[MikaelSmith]

Writable allows changing or submitting content to an existing entry. To create a new entry, we need a new semantic for what it means to create it.

Creating a new file entry with content is probably not much (any?) different than updating an existing one. Creating a new directory probably requires different semantics - it may need configuration that's not present as content.

[MikaelSmith]

Should do #675 first.

[MikaelSmith]

Initial thought (from Enis): plugin interface is an entry configuration struct (JSON object for external plugins) that can have configuration. File creation with file content will be a special case of this.