Improve user settings ui

With the recent addition of new user settings, I felt like the /user-settings page had become a bit messy. This is a quick fix to make it more readable :

• Change h3's to h2's to make them bigger, and also to be consistent with the /settings page
• Add spacing and a separating line between sections (this also impacts the /settings page)

While I was at it, I also fixed the height of the file inputs. They were just slightly vertically off center in Chrome, but a bit more clipped in Firefox.

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Before/after in Chromium (looks better live!):

looks good to me

merged

mentioned in commit 97ae186a