[FIX] theme: improve search bar implementation and behavior

This commit removes the extra search bar that was dedicated to the
mobile layout and adapts the desktop's one to be compatible with mobile.
It also brings back the "Hide Search Matches" button.

extensions/odoo_theme/static/style.scss

@@ -72,17 +72,22 @@ header.o_main_header{
         }
     }
     .o_search_wrapper {
+        display: flex;
         @include media-breakpoint-down(lg) {
-           display: none !important;
+            display: block;
+            position: absolute;
+            width: 100%;
+            padding: 0 10px;
+            top: calc(100% + 30px);
        }
     }
 
     .highlight-link {
         margin-bottom: 0;
-        display: flex;
-        align-items: center;
+        padding: 0 1rem;
+        text-align: center;
+        align-self: center;
         a {
-            padding: 0 1rem;
             @include font-size($font-size-secondary);
         }
     }
@@ -369,6 +374,9 @@ header.o_main_header{
     main {
         position: relative;
         padding-top: 3rem;
+        @include media-breakpoint-down(lg) {
+            padding-top: 7rem;
+        }
         padding-bottom: 3rem;
 
         @include media-breakpoint-up(lg) {