Simplify subnavigation layout

Using `flex` instead of a fixed width for the navigation, the elements
take all the available space when the search form isn't present. That
wasn't the case before and produced a strange effect on medium-sized
screens.

This way we also align the search to the right.

app/assets/stylesheets/layout.scss

@@ -692,21 +692,18 @@ body > header,
 
 .subnavigation {
   @include full-width-background;
-
+  @include grid-column-gutter;
-  @include breakpoint(small only) {
+  display: flex;
-    @include grid-row-nest;
+  flex-direction: column;
-  }
 
   @include breakpoint(medium) {
     background: $body-background;
     color: $text;
+    flex-direction: row;
     padding-bottom: 0;
-  }
 
-  .search-form-header {
+    > :first-child {
-
+      flex-grow: 1;
-    @include breakpoint(small only) {
-      padding-left: rem-calc(map-get($grid-column-gutter, small) / 2);
     }
   }
 
@@ -790,6 +787,11 @@ body > header,
   }
 }
 
+.subnavigation-with-top-links {
+  padding-left: 0;
+  padding-right: 0;
+}
+
 .top-bar-right {
 
   .is-active {

app/views/layouts/_header.html.erb

@@ -33,25 +33,20 @@
         </ul>
 
         <div class="show-for-small-only">
-          <div class="subnavigation row">
+          <div class="subnavigation subnavigation-with-top-links">
             <%= render "shared/subnavigation" %>
-            <div class="small-12 column">
             <%= render "shared/top_links" %>
           </div>
         </div>
       </div>
     </div>
   </div>
-  </div>
 
-  <div id="navigation_bar" class="subnavigation expanded">
+  <div id="navigation_bar" class="subnavigation">
-    <div class="row">
     <div class="hide-for-small-only">
       <%= render "shared/subnavigation" %>
     </div>
-      <div class="small-12 medium-3 column">
+
     <%= yield :header_addon %>
   </div>
-    </div>
-  </div>
 </header>

app/views/shared/_subnavigation.html.erb

@@ -1,4 +1,4 @@
-<nav class="small-12 medium-9 column">
+<nav>
   <ul>
     <%= raw content_block("subnavigation_left") %>
 

spec/system/budgets/investments_spec.rb

@@ -181,7 +181,7 @@ describe "Budget Investments" do
 
       visit budget_investments_path(budget, heading_id: heading.id)
 
-      within(".expanded #search_form") do
+      within "#search_form" do
         fill_in "search", with: "Schwifty"
         click_button "Search"
       end

spec/system/debates_spec.rb

@@ -536,7 +536,7 @@ describe "Debates" do
 
         visit debates_path
 
-        within(".expanded #search_form") do
+        within "#search_form" do
           fill_in "search", with: "Schwifty"
           click_button "Search"
         end
@@ -553,7 +553,7 @@ describe "Debates" do
       scenario "Maintain search criteria" do
         visit debates_path
 
-        within(".expanded #search_form") do
+        within "#search_form" do
           fill_in "search", with: "Schwifty"
           click_button "Search"
         end
@@ -629,7 +629,7 @@ describe "Debates" do
       create(:debate, title: "Abcdefghi")
 
       visit debates_path
-      within(".expanded #search_form") do
+      within "#search_form" do
         fill_in "search", with: "Abcdefghi"
         click_button "Search"
       end

spec/system/proposals_spec.rb

@@ -1119,7 +1119,7 @@ describe "Proposals" do
 
         visit proposals_path
 
-        within(".expanded #search_form") do
+        within "#search_form" do
           fill_in "search", with: "Schwifty"
           click_button "Search"
         end
@@ -1139,7 +1139,7 @@ describe "Proposals" do
 
         visit proposals_path
 
-        within(".expanded #search_form") do
+        within "#search_form" do
           fill_in "search", with: proposal1.code
           click_button "Search"
         end
@@ -1155,7 +1155,7 @@ describe "Proposals" do
       scenario "Maintain search criteria" do
         visit proposals_path
 
-        within(".expanded #search_form") do
+        within "#search_form" do
           fill_in "search", with: "Schwifty"
           click_button "Search"
         end
@@ -1236,7 +1236,7 @@ describe "Proposals" do
       create(:proposal, title: "Abcdefghi")
 
       visit proposals_path
-      within(".expanded #search_form") do
+      within "#search_form" do
         fill_in "search", with: "Abcdefghi"
         click_button "Search"
       end