Refactor gulp/task setup

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• [x] Simplify config
• [x] Simplify CSS build tasks
• [x] Clean up images task
• [x] Simplify JS/Webpack config & tasks
• [x] Clean up SVG sprite task
• [x] Clean up Pug template build task
• [x] Remove unneeded dependencies
• [x] Add docs for npm scripts
• [x] Add lint script for JS & CSS
• [x] Remove old Gulp code

[stormwarning]

Haven't confirmed for sure yet, but looks like I should be able to drop ~20 more devDependencies once this is ready!