Linting

[abisz]

I started setting up the linting config and fixed the errors of html.js.

Before I continue, I wanted to check whether the styling options are approved.
Especially the semicolon option might be controversial, as they aren't used in the gatsby lint options.

IMO it's good practice to write semicolons, but I don't care if you want to stick to the gatsby style.

I'm doing the rest of the files within this pull request, just wanted to check before doing a lot of work.

[alxshelepenok]

Before I continue, I wanted to check whether the styling options are approved.

Yes, I think follow the Gatsby style code does not make sense, if the Airbnb style is defined as the standard.

Initially, I developed the Lumen-starter based on the gatsby-starter-blog.
In the gatsby-starter-blog did not have semicolons, therefore in the Lumen-starter there are none.

But if you follow Airbnb Style Guide (https://github.com/airbnb/javascript/tree/master/react), as in this case, then semicolons are needed to avoid errors.

[abisz]

@WPioneer I finished fixing all linting errors and added some exceptions for rules that would have required more restructuring. IMO this is ready to merge.

[alxshelepenok]

@abisz Great job! Thank you for your contribution!