Use composer state as source of truth for embeds/links on publish

[gaearon]

This is the equivalent of #5595, but for embeds, links, and GIFs.

With this PR, we're not using the old Composer state variables like quote, extLink, or extGif for posting. Instead, we resolve links/records from the composerState managed by the new composer reducer.

This also means that useExternalLinkFetch is no longer used as a source of truth for posting. It's still used for previews though. In a follow up, I'll fix that up.

Note that this is a slight performance regression. That's because resolving is always done from scratch during post, and there is no caching applied. (Effectively, link meta and previews now get fetched twice — once on post and once on publish.) This will be fixed in a follow-up PR where I'll move resolving into RQ and hook it up to both places.

I won't merge this on its own without the follow-up PR that fixes perf. So this is mostly just for isolated review.

Test Plan

Try combinations of quotes, images, GIFs, other records, and link embeds. Verify the posted result makes sense.

[github-actions]

Old size	New size	Diff
7.92 MB	7.92 MB	54 B (0.00%)

---

[haileyok]

Seems fine as is, but leaving a note here in case you wanted to remember to come back here.

edit: lots of other todos up above so assume these are already noted 👍

[gaearon]

this one actually seems like it's a bit more convenient to leave as is, we're relying on initQuote.uri being an at:// uri in a different place

[haileyok]

Tested composer + resulting post with:

• Gif by itself
• Gif with text in post
• Gif with quote (see note below)
• Video by itself
• Video with text in post
• Video with captions
• Video with quote
• Quote by itself
• Quote with text
• Quote with everything above ^

Bugs (not regressions):

• Trying to add a quote or link embed after adding a gif does not work
  • Can try to fix this after merging this stack
• Adding a quote prevents you from adding any other media types
  • The combination of these two makes it impossible to have a post with a gif and a link embed
  • You can add a quote embed before adding a gif, so only links are impossible

Notably though, I think with our current setup you can't have both a gif and a link embed, since they both use the same type in the record. IMO we should address that with a lexicon addition/change but we'll see. However, the quote-gif combo should work correctly so we need to fix that.

[haileyok]

Works well from the testing above ^. A few bugs, only one of which we can actually fix and already exists in prod. That can come in a separate PR. Beautiful!

[haileyok]

Ah, looks like in the final PR that bug is fixed. So really just the weird gif and link thing we can't solve with frontend alone.