[simple theme] rework select; add safesearch to search and replace / with › in article url

* rework selection UI in pref (fix based on: https://github.com/twelsby/searx/commit/78643e9f43a103c523f112e9f3ce26a5c7bb3a0f)
* moved search filters underneath categories
* cut params from url and replace / with ›
* make h3 and url in article bigger
* add safe search select to search filter (this will not override settings and only be valid while on result page in a session)
* make search form button not overlap each other when js is disabled
* 1rem padding around preview image and thumbnail in default article template

searx/static/themes/simple/src/less/index.less

@@ -22,7 +22,8 @@
     visibility: hidden;
   }
 
-  #search {
+  #search,
+  #search_header {
     margin: 0 auto;
     background: inherit;
     border: inherit;