Add stylelint

[Tim Nguyen]

The CSS files should be linted to have a common code style + no errors.

[Madis Otenurm]

I think that would make sense when there is a proper management UI. Common users don't like reading console logs...

[Tim Van den Eynde]

I can at least manually run this before merging to master, so this sounds like a good idea already. I try to manually enforce the same style, but I can definitely overlook something.

I have no experience with CSS linting tools though. I'll look into stylelint later today.

[Tim Van den Eynde]

@Madis0 By the way, a linting tool would only make sense for contributors, not for users. So a console tool would be fine imo.

[Tim Nguyen]

You could make a travis job that disallows merging the PR until all stylelint issues are solved.