Use Twitter OG tags if no Twitter credentials are configured

[M4tthewDE]

Pull request checklist:

• CHANGELOG.md was updated, if applicable

Description

Appends "(bot)" to User-Agent so that the default resolver can fetch OpenGraph tags for twitter links.
I don't like how this identifies twitter links, but it's the best I could come up with on the spot.

Note that the default resolver is only used if no Twitter token is specified in the config.

Closes #499

[codecov]

Codecov Report

Merging #522 (d18bc5b) into master (6cf160d) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
+ Coverage   46.90%   46.91%   +0.01%     
==========================================
  Files          99       99              
  Lines        3710     3717       +7     
==========================================
+ Hits         1740     1744       +4     
- Misses       1920     1922       +2     
- Partials       50       51       +1     

Files Changed	Coverage Δ
internal/resolvers/default/link_loader.go	49.05% <50.00%> (-0.45%)	⬇️
internal/resolvers/twitter/resolver.go	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[KararTY]

Had to look for a bit since I remember paj saying something:

the user-agent must only be added to the twitter requests

You're gonna have to ask pajlada if that's still the case, though.

[KararTY]

Also if I'm not mistaken, #510 is also trying to solve the same underlying issue - The fact Twitter embeds don't work.

I don't know if there has been a discussion whether we'd go that route or this route with OG tags.

[M4tthewDE]

I would consider falling back to OG tags by default to be desirable despite the other PR, since the API used there is unofficial.

[pajlada]

Thank you!