Issues and improvements for search functionality

[LukasKalbertodt]

The backend site of the search story is pretty good, thanks to #383. But, especially in the frontend, there is a lot to do. In random order:

• Probably switch to instant-search.js no
• Use history API in a better way: don't add an entry for every key stroke
• Highlight matches (i.e. somehow highlight the parts of data that matched the query) -> Highlight matches in main search results #1065
• Pagination (?) -> Add pagination or infinite scrolling to main search #1067
• Generally have a better design
• Add button to "get back" to the previous page. Also do that when ESC was pressed
  • Emptying the search input should do the same -> actually it shouldn't
• Update search input field correctly when navigating via browser history stack
• Make easier to navigate with keyboard (e.g. arrow keys?) -> Make it possible to select search results with arrow keys #1066
• Detect when index is not ready to be searched and display a friendly message
• Deal with the index not existing at all (or some other random error) in a better way
• Fix thumbnail always assuming it's a video (in case thumbnail is missing)

[LukasKalbertodt]

All remaining points have separate issues now.