feat(gateway): improved templates, user friendly errors

[hacdias]

Closes #262. This PR does the following:

• Adds a new human-friendly error page when something wrong goes on.
• Reworks templates a bit, making the process to develop faster and easier.
  • Common header file.
  • No more bash scripts or go generate.
  • Cleaned up CSS.
  • Removed twitter and opengraph meta tags and just use standard <title> and <meta description.
• Updated some file icons for the directory to be coherent.
• Removed "Preview as JSON" from DAG template. We removed that function in fix(gateway): undesired conversions to dag-json and friends kubo#9566, but forgot to update the template.
  • Preview for DAG-JSON and DAG-CBOR in feat(gateway): visualize dag-cbor and dag-json #315.
• Removed hardcoded Kubo issues page and replaced by customisable menu items in the config. Each implementation can now add their own links that will show up to the right of "About" and "Install".

As usual, instructions on how to test the templates are in gateway/assets/README.md.

[codecov]

Codecov Report

Merging #298 (ab987b2) into main (86cdb24) will decrease coverage by 1.18%.
The diff coverage is 44.04%.

❗ Current head ab987b2 differs from pull request most recent head 37da207. Consider uploading reports for the commit 37da207 to get more accurate results

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
- Coverage   49.37%   48.20%   -1.18%     
==========================================
  Files         279      281       +2     
  Lines       33598    33646      +48     
==========================================
- Hits        16590    16220     -370     
- Misses      15276    15737     +461     
+ Partials     1732     1689      -43     

Impacted Files	Coverage Δ
gateway/gateway.go	88.52% <ø> (ø)
gateway/handler_block.go	0.00% <0.00%> (-74.36%)	⬇️
gateway/handler_car.go	0.00% <0.00%> (-58.34%)	⬇️
gateway/handler_codec.go	0.00% <0.00%> (-32.95%)	⬇️
gateway/handler_ipns_record.go	0.00% <0.00%> (-20.69%)	⬇️
gateway/handler_tar.go	0.00% <0.00%> (-55.13%)	⬇️
gateway/handler_unixfs__redirects.go	37.63% <0.00%> (+1.07%)	⬆️
gateway/handler_defaults.go	36.66% <11.11%> (+0.24%)	⬆️
gateway/assets/test/main.go	20.37% <26.82%> (ø)
gateway/handler_unixfs_dir.go	63.58% <30.00%> (+2.08%)	⬆️
... and 5 more

... and 13 files with indirect coverage changes

[hacdias]

I'm opening this for review. Please give me feedback regarding error messages and other things.

[BigLep]

Thanks for working on this. A few things:

1. This needs a changelog entry.
2. How does someone customize this for their own needs? 3 immediate examples I see:

• (most important) how does ipfs.io gateway customize this for their product
• (maybe) Kubo default (which can encourage someone to install Kubo for themselves)
• (likely not as important) bifrost-gateway default

[hacdias]

@BigLep

How does someone customize this for their own needs?

Very good question. And I think the answer depends on what you want to customize. What do you need to customize? Right now none of the templates are or have been customizable (directory index, DAG index, and now error). I removed the Kubo hardcoded link and added customisable links for the menu.

[lidel]

@hacdias thank you for cleaning this up 🙏

Mind resolving conflicts and creating a PR draft in Kubo before merging?
If Kubo is green, please merge and rebase #307 so we can continue there 🙏

[hacdias]

Checks are green in Kubo: ipfs/kubo#9904. Merging.