Move new investment button styles to CSS files

app/assets/stylesheets/budgets/investments/form.scss

@@ -10,4 +10,9 @@
     padding-left: 0;
     padding-right: 0;
   }
+
+  [type="submit"] {
+    @include regular-button;
+    width: 100%;
+  }
 }

app/components/budgets/investments/form_component.html.erb

@@ -96,6 +96,6 @@
   <% end %>
 
   <div class="actions small-12 medium-6 large-4">
-    <%= f.submit(nil, class: "button expanded") %>
+    <%= f.submit %>
   </div>
 <% end %>