Feature/pc 25 linting

[CuriousCactus]

Added linting and did some refactoring:

• Added sass-lint and fixed errors
• Added eslint and fixed most errors
• Updated some packages
• Moved the SVGs and made them load with webpack, rather than just placing them in the public folder (This was needed because an update to react-scripts stopped them loading from there. The update to react-scripts was needed because of an eslint dependency conflict.)

Not done yet:

• Making the linting run as part of any pipeline. At the moment it can be run from the command line using yarn gulp js:lint or yarn gulp sass:lint
• Fixing "'thing' is missing in props validation" linting error, either by ignoring the errors or adding checking (see https://reactjs.org/docs/typechecking-with-proptypes.html)

[prophet-kn]

Why is package-lock.json deleted?

[CuriousCactus]

Mostly because it's a draft, but I do think we should pick a package manager and delete the other lock file at some point. No strong opinions on which.

[prophet-kn]

We will stick to yarn as it's done by the same people that built react and yarn is not controlled by venture capitalists. :)