Add basic linting to fn ui

[vzDevelopment]

This PR adds basic linting to the Fn UI providing a consistent code style across the repo. It uses eslint's recommended rules to find common problems. I have fixed everything eslint is currently complaining about so it now passes.

I was thinking it'd also be nice to incorporate this with CircleCI so that it's maintained. This might also help find some bugs before the code review. What do you all think about this?

Testing

To test this I've:

• run npx eslint . in the repo and fixed all the errors so there shouldn't be any when you run this (make sure eslint is installed locally first by running npm install)
• checked functionality around the interface (in particular around lines which I have changed) to ensure I haven't broken anything e.g. where I have removed unused variables.

[rdallman]

I was thinking it'd also be nice to incorporate this with CircleCI so that it's maintained. This might also help find some bugs before the code review. What do you all think about this?

that would be great.

[rdallman]

thanks @vzDevelopment !