Remove unneeded optgroup in locale switcher

The `<optgroup>` doesn't make much sense if all options are inside one
group. And the information provided was redundant: when using a select
field having "Language" as a label, it's obvious that the options are
the available languages.

Now that, since now the `<select>` field is smaller, we need to add an
extra padding so the icon doesn't overlap the text.

app/assets/stylesheets/layout.scss

@@ -1012,7 +1012,8 @@ footer {
     margin-bottom: 0;
     margin-top: $line-height / 4;
     outline: none;
-    padding: 0 $line-height / 4;
+    padding-left: $line-height / 4;
+    padding-right: calc(#{$line-height / 4} + 1em);
     width: auto;
 
     &:focus {

app/views/shared/_locale_switcher.html.erb

@@ -5,14 +5,12 @@
         <%= t("layouts.header.locale") %>
       </label>
       <select class="js-location-changer locale-switcher inline-block" name="locale-switcher" id="locale-switcher">
-        <optgroup label="<%= t("layouts.header.available_locales") %>">
         <% I18n.available_locales.map do |loc| %>
           <option <%= "selected" if loc == I18n.locale %>
                   value="<%= current_path_with_query_params(locale: loc) %>">
             <%= name_for_locale(loc) %>
           </option>
         <% end %>
-        </optgroup>
       </select>
     </form>
   </div>

config/locales/en/general.yml

@@ -226,7 +226,6 @@ en:
     header:
       administration_menu: Menu
       administration: Administration
-      available_locales: Available languages
       collaborative_legislation: Collaborative legislation
       debates: Debates
       locale: "Language:"

config/locales/es/general.yml

@@ -226,7 +226,6 @@ es:
     header:
       administration_menu: Menú
       administration: Administración
-      available_locales: Idiomas disponibles
       collaborative_legislation: Legislación colaborativa
       debates: Debates
       locale: "Idioma:"