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)

tests/e2e/utils_e2e.ts

@@ -105,6 +105,7 @@ export async function save_visual(page: Page) {
         page.locator('#repo_migrating'),
         // update order of recently created repos is not fully deterministic
         page.locator('.flex-item-main').filter({hasText: 'relative time in repo'}),
+        page.locator('#activity-feed'),
         // dynamic IDs in fixed-size inputs
         page.locator('input[value*="dyn-id-"]'),
       ],