Show number of files in snippets list

What does this MR do?

Adds a file count statistic to the snippets list. Hovering over the file count shows a tooltip (see screenshot below). If the snippet statistics are pending computation, the file count is not shown.

Screenshots (strongly suggested)

• The first snippet has a file count of 1. An appropriate tooltip is also available.
• No file count is shown for the second snippet because its statistics are not available yet.

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #215043 (closed)