Use relative URLs where possible across all the app

Skipping API responses, where we need absolute URLs, and those that are
intended for sharing or external use:

- account/join_codes/show.html.erb - Join code URL for sharing
- boards/edit/_publication.html.erb - Publication URL for sharing
- public/* views - Public page URLs and og:url meta tags
- pwa/manifest.json.erb - PWA manifest needs absolute URLs

For this, we had to replace `url_for` used with Active Storage variants
and previews with the specific path helper (for Active Storage
representations).

app/views/users/show.html.erb

@@ -41,7 +41,7 @@
       <% if Current.user == @user %>
         <hr class="separator--horizontal full-width flex-item-grow margin-block-start-double" style="--border-color: var(--color-ink-light);" aria-hidden="true">
 
-        <%= button_to session_url(script_name: nil), method: :delete, class: "btn txt-x-small center", data: { turbo: false } do %>
+        <%= button_to session_path(script_name: nil), method: :delete, class: "btn txt-x-small center", data: { turbo: false } do %>
           <span>Sign out of Fizzy on this device</span>
         <% end %>
       <% end %>