Highlight line which includes search term is code search results

What does this MR do?

Highlight line which includes search term is code search results.

There's also some existing UX proposals in #16429 (closed):

To help users read the results we give them I propose we add line highlighting + bold key word highlighting to those results.

I've implemented the line highlighting as the smallest valuable change, because highlighting specific search term occurrence within line is more tricky and error prone.

Screenshots

Obtained from a fresh development copy (logged as root user):
http://localhost:3000/search?utf8=%E2%9C%93&snippets=&scope=&repository_ref=master&search=anonymizeIp&project_id=8

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

Need help to test with Internet Explorer, Microsoft Edge and Safari, because they are not available in my operating system.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• • Google Chrome
• • Mozilla Firefox
• • [-] Internet Explorer
• • [-] Microsoft Edge
• • [-] Safari

Security

No need for security review.

Closes #196384 (closed)