Sync post

[szymonchudy]

Hey! Happy to bring the first PR here 💪

This built on top of @crutchcorn's #74 mostly following conventions set there. Not a lot of dependencies, though.

Summary

• Add sync-post processor for syncing posts from GitHub to S3/DB
• Support standalone posts and collection chapters
• Handle multiple locales and authors
• Extract shared extractLocale util
• Propose renaming of createBucket → ensureBucket for a better clarity; can be easily reverted cd88eb7

Open questions

1. Image handling is not covered in this PR as it wasn't part of the issue, but I guess we should handle it as well, right? What are the requirements for images embedded in markdown posts?
   • Should they be uploaded to S3?
   • Any processing needed (resize/optimize)?
   • URL rewriting in markdown?
2. Should it be included in this PR or a follow up?
3. What's the purpose of collection filed in PostInfo? Looks redundant now.

[fennifith]

This looks great! Left a few comments :)

To answer your questions:

• Yes, images will need to be handled! That's a good callout.
  • Rewriting the URLs in markdown on the server seems brittle, as the image handling is closely tied to the frontend
  • It might be better to broadly define "post attachments" as any file adjacent to the post (there are some examples of pdf/pptx files in the repo - and this would also be useful for #29) and sync those with S3
• I don't think that should go in this PR - I've started detailing #86 for this :)
• The collection field definitely isn't needed in the frontmatter, but it is still used by the frontend (it just gets populated elsewhere).