Limit max index size for Snippet content

The CE port is at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31883

What does this MR do?

To avoid indexing timeout. 1 MB is chosen because it is repo index limit per file, and snippet will soon be migrated from database to repo.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

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

Related to #12111 (closed)