Use text instead of IDs in feature specs

This way we write the tests from the user's point of view: users can see
(for example) a proposal with the title "Make everything awesome", but
they don't see a proposal with a certain ID.

There are probably dozens, if not hundreds, of places where we could
write tests this way. However, it's very hard to filter which ones are
safe to edit, since not many of them have an HTML class we can use in
the tests, and adding a class might generate conflicts with CSS styles.

So, for now, I'm only changing the ones allowing us to cleanly remove
useless assignements while maintaining the code vertically aligned.

app/views/admin/budget_headings/index.html.erb

@@ -20,7 +20,7 @@
     </thead>
     <tbody>
       <% @headings.each do |heading| %>
-        <tr id="<%= dom_id(heading) %>">
+        <tr id="<%= dom_id(heading) %>" class="heading">
           <td><%= link_to heading.name, edit_admin_budget_group_heading_path(@budget, @group, heading) %></td>
           <td><%= @budget.formatted_heading_price(heading) %></td>
           <td><%= heading.population %></td>