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.

---

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

src/components/settings/settings.vue

@@ -57,7 +57,10 @@
 @import '../../_variables.scss';
 
 .setting-item {
+  border-bottom: 2px solid var(--btn, $fallback--btn);
   margin: 1em 1em 1.4em;
+  padding-bottom: 1.4em;
+
 
   textarea {
     width: 100%;