Update Frontend doc with new standards and tools

With the recent changes we are missing some helpers/rules/best practices/styleguide in out Frontend docs, this issue keeps track of all:

Webpack

• - Use import foo from 'bar' and export default instead of require() and instead of assigning to global namespace
• - No need to use .es6 anymore
• - No need for IIFE anymore
• - How to handle relative paths: ~ vs ../../ vs create an alias for each main folder

Vue

• v-on versus @ syntax - the latest
• v-bind versus : - the latest
• " versus ' in templates - the first
• data should be a function
• declare props as an object {}
• order of the methods of a component, example of airbnb for react - https://github.com/airbnb/javascript/tree/master/react#ordering
• declare a component as a plain object
• <component></component> versus <component/>
• How to import components - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9855/diffs#note_25231856

CSS

• Don't use IDs
• Reuse color variables, don't add new ones unless it's super necessary
• Try not to use !important

Common/General

• No data mutation
• Prefer pure functions
• No constructor side-effects
• Use of babel-polyfill, specifically with commonjs https://github.com/zloirock/core-js#commonjs
• How to parse strings into numbers, I've seen in our code base 3 ways of doing this: +string, Number(string), parseInt(string, 10)- we should choose one

@gl-frontend Feel free to add things to this issue and to check the ones that are already documented! 💪