[admin] Streamline form inputs

[elia]

Summary

• Depends on [admin] Misc. bugfixes #5386

Switches from using rails from builder helpers directly to raw components with higher level constructors that can rely on the form builder. This change brought in a considerate amount of simplification.

The ui/forms/input component is a candidate for splitting later on, most of the styles were shared between the input, select, and textarea tags, but it will be under scrutiny for a split while we progress with building other pages.

A bunch of the smaller UI details have been fixed (colors, spacing, etc.).

A basic version of the multi select is available but still not intended as the final solution, for which we'll need to find/build a substitute to select2.

Finally preview playgrounds were setup separately for input, select, and textarea`:

Checklist

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[codecov]

Codecov Report

Merging #5385 (f6d9cf2) into nebulab/admin (f4c0dc2) will decrease coverage by 0.10%.
The diff coverage is 100.00%.

@@                Coverage Diff                @@
##           nebulab/admin    #5385      +/-   ##
=================================================
- Coverage          88.87%   88.78%   -0.10%     
=================================================
  Files                615      606       -9     
  Lines              14861    14670     -191     
=================================================
- Hits               13208    13025     -183     
+ Misses              1653     1645       -8     

Files Changed	Coverage Δ
...mponents/solidus_admin/ui/forms/field/component.rb	100.00% <100.00%> (ø)
...mponents/solidus_admin/ui/forms/input/component.rb	100.00% <100.00%> (ø)

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[rainerdema]

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