[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/gruntfile.js

@@ -108,7 +108,6 @@ module.exports = function (grunt) {
           'js/searxng.head.min.js': ['src/js/head/*.js'],
           'js/searxng.min.js': [
             'src/js/main/*.js',
-            '../__common__/js/*.js',
             './node_modules/autocomplete-js/dist/autocomplete.js'
           ]
         }