Resolve "Autocomplete dropdown for usernames shows HTML if you attempt to search for a username that starts with a period"

What does this MR do?

This fixes the issue described in #60552 (closed) by escaping the period (.) character when being highlighted in the dropdown for user tagging.

It does so by overriding the default highlighting from At.js which just escaped the + character. Theoretically, you could escape all regex special characters here, but I don't think that's necessary right now.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #60552 (closed)