Add QA checks to ensure code quality

[pandafy]

Feature description

While working on #617 I encountered that some files have trailing white spaces. I have configured my code editor to automatically trim while spaces. Since white spaces can be added by anyone and could sneak through reviews, I recommend adding QA checks to make sure they are absent and does not affect code quality.

[MattIPv4]

+1 to this, we should be linting html/css

[naomi-lgbt]

Ooooh can I poke at this? I'm addicted to GitHub actions 😅

[MattIPv4]

I'm not sure what the best practice is, but I imagine rubocop can lint .html.erb files directly, and then yeah, we'll need a new job (& docs) for running sass-lint or w/e

[MattIPv4]

But yes, go for it!