Fix mentions demo to behave more consistently

[TheSpyder]

Ticket: n/a, discovered by @lostkeys in slack

Site: Staging branch

Changes:

• Sort the list of usernames on load. This doesn't guarantee a sorted result, because we use an object store, but I don't really want to sort on every search
• Fix the case insensitive name match code, which was only lowercasing the query term. The result was what a user would consider completely unpredictable behaviour

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• modules/ROOT/nav.adoc has been updated (if applicable).
• Included a release note entry for any New product features.
• If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc.

Review:

• Documentation Team Lead has reviewed

[kemister85]

Thank you for this fix.

[LewisAtTiny]

Nice change!!