Optimize build time with hard-source-webpack-plugin

[nblthree]

Summary Closes #444, closes #425

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Docs
• [x] Build-related changes
• [ ] Other, please describe:

If changing the UI/CSS related code, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• [x] Briefly describing what this PR does in the title in Angular Commit Message Conventions, e.g. fix: rebuild when a page is added

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information: The log

yarn run v1.19.0
$ node ../packages/saber/lib/cli.js
[info] Using config file: saber-config.js
[info] Using theme: ./src
[info] Using 9 plugins from config file
[hardsource:21992d5c] Using 10 MB of disk space.
[hardsource:21992d5c] Tracking node dependencies with: yarn.lock.
[hardsource:21992d5c] Reading from cache 21992d5c...
[hardsource:d4665d8a] Using 10 MB of disk space.
[hardsource:d4665d8a] Tracking node dependencies with: yarn.lock.
[hardsource:d4665d8a] Reading from cache d4665d8a...
[success] Compiled client successfully in 2.9s!
[info] Available at http://localhost:3000
[info] 73.2 MB memory used
[success] Compiled server successfully in 5.2s!
[info] (10:57:21) Rebuilding client due to changes in ~\issuehunt\saber\website\.saber\routes.js
[info] (10:57:21) Rebuilding server due to changes in ~\issuehunt\saber\website\.saber\routes.js
[hardsource:21992d5c] Using 10 MB of disk space.
[client] 62.50% building 7/8 modules 1 active ...style&index=0&id=2a183b29&scoped=true&lang=css&
[client] 94.50% after seal
[success] Compiled client successfully in 6.3s!
[info] Available at http://localhost:3000
[info] 141 MB memory used
[success] Compiled server successfully in 6.6s!

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