[Rails5] Fix snippets_finder arel queries

What does this MR do?

There is a bug in Rails 5.0 and 5.1 in Arel/ActiveRecord.
This MR converts arel based queries to their raw counterparts to make the finder work in Rails 5.0.

These changes should be reverted when we're on Rails 5.2 as since that version such queries work well again.

We've already faced this problem before.

Are there points in the code the reviewer needs to double check?

Check if SQL queries are OK.

Why was this MR needed?

Migration to Rails 5.0.

Screenshots (if relevant)

No.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47832 (closed)