Many small tooling cleanup tasks

[ErisDS]

This issue contains a list of mostly small cleanup tasks for our tooling.

Aim is to move towards consistency between repos (both public and private) to lean on package.json scripts more heavily where possible, simplify grunt & essentially make this repo neater & easier to use 😁

• Switch eslint to use eslint-plugin-ghost (OK this isn't that small)
• Use .js files rather than .json for eslint config
• Remove grunt-eslint, use grunt run or grunt shell instead cos it's more generic, and we can do other things this way
• Add lint as a script in package.json
• Swap .gitignore for standardised file + customisations at end
• Fix .gitignore so we don't have tracked files in in the ignore:

content/data/README.md
content/themes/casper/
content/apps/README.md
content/images/README.md
content/logs/README.md
core/server/public/ghost-sdk.min.js
core/server/web/admin/views/.gitkeep

• Review .npmignore file
• grunt release creates a zip with contents which do not equal .tgz (e.g. 0db01a0)
• Refactor Gruntfile.js so it isn't so indented
• We aren't using greenkeeper - cleanup ignores from package.json
• Why is minimist a dev dep?!
• Fix brute-knex dependency
• Consider moving coverage out of grunt into a script in package.json
• Consider tests: can we restructure them so they are easier to run? 🤔
• Let grunt master fail when working tree is not clean

[ErisDS]

Some of this is outdated, but I'm sure there are plenty more small tasks like this that have cropped up in the year since I wrote this.