Fix width of Search button in sidebar

Whoops, this was working fine in Firefox on Linux but it seems like the
Search button has been very wide on some other platforms/browsers.

tildes/scss/modules/_sidebar.scss

@@ -21,6 +21,10 @@
   .form-search {
     margin-top: 0;
 
+    .btn {
+      width: auto;
+    }
+
     .form-input {
       max-width: unset;
     }