Rework user profile settings

Accessibility:

- improved semantic layout
- Fixes unlabelled input for custom pronouns. CC @hazy
- Adds labels to dropdowns.
- Shortens certain texts for less verbose screen reader outputs and
  people with slow reading speed.
- Turned optional username rename helper text with low contrast into
  "normal" help text.

UI/UX:

- Removes section about primary email which is no longer managed in the
  profile section.
- Fixes section about primary email not displaying in user settings when notifications are
  not available.
- Removes primary email display, because it is not actually a form
  element here. (Alternatively, we could display it and link to the
account settings for managing the email)

(cherry picked from commit 2885ea8da2)

web_src/css/form.css

@@ -1,5 +1,5 @@
 fieldset {
-  margin: 0.5em 0 1em;
+  margin: 0.2em 0 0.3em;
   padding: 0;
 }
 
@@ -8,6 +8,12 @@ fieldset legend {
   margin-bottom: 0.75em;
 }
 
+fieldset + fieldset > legend {
+  width: 100%;
+  padding-top: 1em;
+  border-top: 1px solid var(--color-secondary);
+}
+
 fieldset label {
   display: block;
   margin-bottom: 0.6em;

web_src/css/user.css

@@ -143,7 +143,3 @@
 .notifications-item:hover .notifications-updated {
   display: none;
 }
-
-#pronouns-dropdown, #pronouns-custom {
-  width: 140px;
-}