[enh] introduce categories_as_tabs

Previously all categories were displayed as search engine tabs.
This commit changes that so that only the categories listed under
categories_as_tabs in settings.yml are displayed.

This lets us introduce more categories without cluttering up the UI.
Categories not displayed as tabs  can still be searched with !bangs.

searx/settings.yml

@@ -81,12 +81,6 @@ ui:
     simple_style: auto
     # Open result links in a new tab by default
     # results_on_new_tab: false
-    # categories_order :
-    #   - general
-    #   - files
-    #   - map
-    #   - it
-    #   - science
 
 # Lock arbitrary settings on the preferences page.  To find the ID of the user
 # setting you want to lock, check the ID of the form on the page "preferences".
@@ -233,6 +227,18 @@ checker:
         result_container:
           - has_infobox
 
+categories_as_tabs:
+  - general
+  - images
+  - videos
+  - news
+  - map
+  - music
+  - it
+  - science
+  - files
+  - social media
+
 engines:
   - name: apk mirror
     engine: apkmirror