Prevent servers being slammed from many search suggestion requests

[LordZardeck]

Description

Currently when a customer is typing in the search suggestion box, every character that is typed is immediately sent to the server. Typical auto-complete boxes will have a delay before sending the request to ensure this doesn't happen. This PR simply uses underscore's debounce method to throttle the requests with a default amount of 300ms, following the default configuration of jQuery's delay method: https://api.jqueryui.com/autocomplete/#option-delay

Fixed Issues (if relevant)

1. Search suggestions result in dozens of queued AJAX requests community-features#71

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @LordZardeck. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
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[magento-engcom-team]

Hi @LordZardeck. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.