Support searching against certain entities only

[CHTJonas]

The search page at https://www.camdram.net/search throws a 500 when no search query string is appended. We should look at adding some kind of meaningful content, or redirect, in this situation.

I thinking maybe an advanced search page that lets the user search keywords in the same way as the current search box, but allows them to choose which entities to search or what time frame to search.

https://sentry.io/camdram/camdram/issues/647436713/

Elastica\Exception\ResponseException: [multi_match] unknown token [VALUE_NULL] after [query]
  File "src/Acts/CamdramBundle/Controller/SearchController.php", line 44, in searchAction
    $resultSet = $search->search($query);
  File "web/app.php", line 28
    $response = $kernel->handle($request);
...
(11 additional frame(s) were not displayed)