DRY up the titles between begin-magic in the source docs and posts.json

[narthur]

### Desiderata
- [x] Clarify the desired end state: titles in source docs or in posts.json
- [ ] Temporarily fail build if title mismatch between posts.json & etherpad
- [ ] Ask Danny which title to go with for any conflicts
- [ ] Entirely remove title field from posts.json
- [ ] Fail build if title ever specified in posts.json

Cognata

• 382

• 371

Verbata: DRY, titles of blog posts in two places,

[narthur]

@dreeves You mentioned here that moving away from using begin-magic to set post titles could be a future task. This would actually be quite easy for us to do since we already have the code to pull and parse the existing titles, so writing a script to take all those titles and add them to posts.json would be trivial. Can we get the go-ahead to do this?

[dreeves]

It's the other way around. The markdown rendering for etherpad (aka expost) needs the titles; they can't live in posts.json. DRYing this up means getting the titles out of posts.json.

As a sanity check, can we get the list of any titles that don't already match?

[dreeves]

Or break the build if they don't match -- that's a simple way to see which ones don't match and is a nice failsafe. Of course that failsafe will be moot when we DRY this up, but that's fine.

Relatedly, I may have changed my mind about every field needing to be explicit in posts.json. Candidate exceptions:

1. title is fetched from the source doc's begin-magic (this gissue) and shouldn't be in posts.json at all
2. disqus_id defaults to slug if not present
3. redirects defaults to [] if not present

[narthur]

Decision:

• entirely remove from posts.json
• fail build if title ever specified in posts.json
• sanity check with @dreeves during transition if there are any existing mismatches