Load Tweets using embed endpoint if no API key is present

[Nerixyz]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

If no Twitter API key is configured, the twitter resolver uses the embed endpoint (used by vercel/react-tweet as well). The only downside is that retweets aren't displayed.

[codecov]

Codecov Report

Merging #510 (7d64ecc) into master (88b86e1) will increase coverage by 1.29%.
The diff coverage is 75.59%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
+ Coverage   46.90%   48.19%   +1.29%     
==========================================
  Files          99      100       +1     
  Lines        3710     3872     +162     
==========================================
+ Hits         1740     1866     +126     
- Misses       1920     1954      +34     
- Partials       50       52       +2     

Files Changed	Coverage Δ
internal/resolvers/twitter/tweet_loader.go	36.89% <0.00%> (-0.37%)	⬇️
internal/resolvers/twitter/embed_loader.go	68.22% <68.22%> (ø)
internal/resolvers/twitter/initialize.go	100.00% <100.00%> (ø)
internal/resolvers/twitter/resolver.go	100.00% <100.00%> (ø)

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[KararTY]

Just some info: Went on for a bit trying to find info about the endpoint, vercel/react-tweet#76

Seems like it's an internal API used by Twitter's external iframe/HTML embeds.

[pajlada]

I prefer the solution from #522 right now - the embed solution doesn't seem to work which tells me it's going to be a bit flaky. If there's a library we can use that keeps this up-to-date I'm open to using this approach