Add hard source webpack plugin in experimental speed mode

[maier49]

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
• schema.json has been updated appopriately

Description:
Adds the hard source plugin to improve repeated build times. Performance improvements are tempered by the necessity of excluding the mini-css-extract-plugin, but still speeds up repeated builds by 20% or more.
Related to #217

[matt-gadd]

we should probably have a discussion re: experimental speed mode as ideally we'd promote that to the default in v8

[maier49]

After further testing for #428 it turns out after the initial drop build times would gradually increase on each subsequent build until they were longer than the original time. It's probably an interaction with some other feature we're using but unless we get that figured out this wouldn't be good to merge.