Curriculum edits

I've made some edits to the frontend curriculum based on my experience teaching it last month. It's nothing radical, mostly just some re-wording and and re-structuring of a few pages.

I don't know if there's anything like a review process for changes like this, but I'm happy to discuss them.
I'd really prefer 'hash' here instead of 'hashtag', though this is probably a linguistic battle I will eventually lose.