Search: Counts endpoint respond with 408 for query timeouts

What does this MR do?

Related to #334961 (closed)

Detect and respond to the search/count endpoint with a 408 Request Timeout response code and empty JSON response if an ActiveRecord::QueryTimeout occurs.

Screenshots or Screencasts (strongly suggested)

When timeouts occur the counts will not load as shown below

How to test

I've provided a small patch that will force the search count queries to timeout.

1. Download the patch force_count_timeout.txt
2. Fetch and checkout this branch git fetch origin 334961-search-counts-endpoint-instead-of-responding-with-500-respond-with-408-request-timeout & git checkout 334961-search-counts-endpoint-instead-of-responding-with-500-respond-with-408-request-timeout
3. Copy the entire file contents from the patch you downloaded in step 1
4. Run the command pbpaste | git apply from your /gitlab directory - This should add some local code changes
5. Navigate to Global Search http://127.0.0.1:3000/search and search something
6. All of the count tab calls should timeout, but if it doesn't try a few more times/changing tabs/etc.
7. You can verify the timeouts using the developer tools or the development.log

Does this MR meet the acceptance criteria?

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Security

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