Upgrade all Webpack usage to Webpack 3

[Reinmar]

Release notes: https://medium.com/webpack/webpack-3-official-release-15fd2dd8f07b

Interesting to check:

• Module concatenation – we should check it in ckeditor5-build-classic. Hopefully, it will improve the bundle size. As for Webpack use in other places... I think that we could also introduce this plugin, but I'd check if it doesn't affect the performance. The performance will count the most in automated tests, so in manual tests we can lose more. Ideally, we should have the same configuration everywhere so we test with the production setup.
• "Better Build Caching" and "Faster initial and incremental builds" sounds like there might be some changes in the memory consumption – I hope that not for worse.

[pomek]

Bundle size with and without module concatenation:

• with: 381307B (372K),
• without: 420550B (411K).

So we could use it.

[Reinmar]

Nice! And gzipped size (which is the one which matters)?

[pomek]

Gzipped size:

• with: 101216B (99K),
• without: 112131B (110K).

[pomek]

PRs:

• Added Module concatenation plugin to webpack ckeditor5-build-classic#28,
• Added Module concatenation plugin to webpack ckeditor5-build-inline#2,
• Added Module concatenation plugin to webpack ckeditor5-build-balloon#2.