Optimise generated CSS

[agubler]

Type: feature

The following has been addressed in the PR:

• There is a related issue
• All code has been formatted with prettier
• Unit or Functional tests are included in the PR

Description:

Uses optimize-css-assets-webpack-plugin to optimise the built css, especially useful to remove duplicates created by inlining css variables.

Resolves #63

[codecov]

Codecov Report

Merging #195 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #195      +/-   ##
==========================================
+ Coverage    52.7%   52.78%   +0.08%     
==========================================
  Files          10       10              
  Lines         573      574       +1     
  Branches      140      140              
==========================================
+ Hits          302      303       +1     
  Misses        271      271

Impacted Files	Coverage Δ
src/base.config.ts	22.98% <100%> (+0.48%)	⬆️

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[tomdye]

Actually, looking at the default preset I think we may want to disable
discardEmpty and possible calc (depending on what it does with variable calculations`.
Discard empty is risky as it could remove empty classes we're relying on for theming etc.

[agubler]

@tomdye I agree with calc we should disable this but remove empty should be fine as at this point it is building the application and the empty selectors are no longer needed as a hook into the theming system.

[agubler]

@tomdye disabled calc