Fix lint failures

Closed Gregory John Martin requested to merge github/fork/rhelmer/fix-lint-failures into master

Created by: rhelmer

I've found and fixed a few eslint errors, @bchivers-stanford the only part I am not 100% sure about is the adCssSelectors in the page-ads.js content script - I'm presuming that ad_css_selectors.js is imported implicitly somewhere?

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