[simple theme] replace Image_layout.js with flexbox CS impl.

* drop image_layout.js from simple theme
* move image_layout.js to oscar theme and delete common js dir (since its empty now)
* align top position of image detail modal with bottom position of search header
* use flexbox to display images; row height can be set via @results-image-row-height in defenitions.less
* display span title underneath each image with a max width of 12rem
* increase margin and padding around image article on desktop and tablet
* make article height smaller on phone layout (height of 6rem) to display more content on current view
* remove content from result, if the title and content matches
* use a group that cotains the flex image article, if images are mixed with other categories
* fix pylint issues in webapp.py
* use the default.html result template in unit tests (thanks @return42)

searx/static/themes/simple/src/less/style-rtl.less

@@ -83,6 +83,11 @@
   }
 }
 
+// Image flexbox
+#main_results div#results.only_template_images #urls {
+  direction: rtl;
+}
+
 // Image detail
 #results.image-detail-open article.result-images[data-vim-selected] .detail .result-images-labels p {
   direction: rtl;