Fixes aria issues on search form (#2211)

* Fixes aria issues on search form
* Swapping aria-label for <label> tag in search
* Removing background gray caused by adding <form> tag to search
* Removing redundant space
* Making form not submit if key is enter

Close #2180

_sass/minimal-mistakes/_search.scss

@@ -50,6 +50,11 @@
     @include breakpoint($x-large) {
       max-width: $max-width;
     }
+
+  }
+
+  &__form {
+    background-color: transparent;
   }
 
   .search-input {
@@ -119,6 +124,7 @@
   font-style: normal;
   text-decoration: underline;
 }
+
 .archive__item-excerpt .ais-Highlight {
   color: $primary-color;
   font-style: normal;