Elasticsearch index snippet description

What does this MR do?

Index snippet description as part of the title search.

This works without schema change because description column already is a field we index (just from a different model). We can live with the fact that existing snippets won't have this indexed and users can always re-index from scratch if they want that.

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

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

Ref #207355 (closed)