[simple theme] add logo to search form on result page

* rework search form as grid layout
* remove various paddings and margins
* add logo and link to index to search form
* make categories bigger on phone

searx/templates/simple/categories.html

@@ -1,4 +1,4 @@
-<div id="categories">{{- '' -}}
+<div id="categories" class="search_categories">{{- '' -}}
     <div id="categories_container">
         {%- for category in categories -%}
         <div class="category"><input type="checkbox" id="checkbox_{{ category|replace(' ', '_') }}" name="category_{{ category }}"{% if category in selected_categories %} checked="checked"{% endif %}/><label for="checkbox_{{ category|replace(' ', '_') }}" class="tooltips">{{ _(category) }}</label></div>