The tab icon names are currently hard coded in the templates.
This commit lets us introduce an icon property in the future, e.g:
categories_as_tabs:
general:
icon: search-outline
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.
