Add autoprefixer to css build

[silverwind]

- node_modules is now vendored (second commit), making it easier for contributors to run the css build as well as ensuring everyone uses the same node dependency versions.

- added make css target. Less to type and the old target is still available and will print a deprecation warning.

- added make update-node-modules which will update all node dependencies and remove unnecessary files in node_modules.

• added autoprefixer which will automatically add CSS vendor prefixes based on a defined list of supported browsers. Removed all relevant prefixed rules from our rules.

Edit: The PR previously contained some controversial changes, removed them.

[codecov-io]

Codecov Report

Merging #6029 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6029      +/-   ##
==========================================
+ Coverage   38.72%   38.72%   +<.01%     
==========================================
  Files         339      339              
  Lines       49013    49013              
==========================================
+ Hits        18980    18982       +2     
+ Misses      27287    27284       -3     
- Partials     2746     2747       +1

Impacted Files	Coverage Δ
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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[silverwind]

One rather important consideration when vendoring node_modules is that if a module or one if its dependencies includes a native compiled (and platform-specific) module (identified by a .node file anywhere in node_modules), this approach will not be portable. Currently this is not the case, but it may in the future. I'm happy to remove the vendoring if people disagree.

[adelowo]

I honestly don't think it makes sense to vendor node_modules. We can leave the newer make commands but vendoring ? Not so sure.

By the way, see
https://github.com/golang/go/blob/ca7c12d4c9eb4a19ca5103ec5763537cccbcc13b/src/cmd/go/internal/modfetch/codehost/codehost.go#L31

go modules have a limit of 500MiB. I am not certain if 500MiB is for just Go source files or the entire repository size. This might be something to consider

[lafriks]

I would also prefer to not have node_modules in git

[silverwind]

I just thought it might be appreciated because the golang modules are vendored too, but I guess I'll undo it. Not a huge fan of it myself because of the mentioned native module issue.

[silverwind]

Reworked PR, now only includes autoprefixer. The make css change is left out as deserves its own PR.