Allow admin to search for user by secondary email address

What does this MR do?

1. Adds ability for admin to search users by secondary emails in /admin/users page
2. Improves parameter naming in Admin::UsersController#index

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

It's my very first MR to gitlab and to open-source project ever, you'd better double check everything, though there are not many changes. Despite I read the guides, I might miss something.

Why was this MR needed?

1. This feature proposed in #23761 (closed)
2. This idea popped up during this MR code review !7130 (merged) !7132 (closed) !

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23761 (closed)