🔨 Refactor fast track & add origins

[Marigold]

This PR was originally only refactoring, but I ended up adding origins as well. Let me know if this is too hard to review and I can split it in two.

Refactoring Fast-track

Simplify fast-track. Allow only one source / origin and get rid of source's short_name (which has caused just confusion in the past). No one has used multiple sources so far. Having just one source simplified the logic a lot. It's all backward compatible.

Add origins

Add new sheet origins_meta. Here's an example of the new template (note that I'll have to update template after merging this to avoid breaking old fast-track spreadsheets). It's very bare-bones at the moment. I'll polish it when we start using it.

TODO

• test loading from CSV
• test loading from S3 path
• add origins
• make sure as much metadata as possible is saved in Origin YAML
• modify fasttrack template to allow only one source / origin

Edits to template spreadsheet

• remove all sources fields
• add origins sheet
• add CHANGELOG sheet with updates

[pabloarosado]

Quick note: I just swapped the values of producer and citation_producer in the template.

[pabloarosado]

I haven't tried it, but it looks fine.