Fix #4747: BUR layout partly broken on iOS.

Move the BUR help text from the <textarea> placeholder attribute to a
<details> tag that embeds the [[help:bur_notice]] wiki page.

Also update some CSS for the <details> tag that was meant to only apply
to the user upgrades page and didn't look good here.

app/javascript/src/styles/common/utilities.scss

@@ -42,6 +42,8 @@ $spacer: 0.25rem; /* 4px */
 .space-x-4 > * + * { margin-left: 4 * $spacer; }
 .space-y-4 > * + * { margin-top: 4 * $spacer; }
 
+.divide-y-1 > * + * { border-top: 1px solid var(--divider-border-color); }
+
 .align-top { vertical-align: top; }
 
 .flex-auto { flex: 1 1 auto; }