Re-name Snippet App to Snippet Show

What does this MR do?

In anticipation of the refactoring of Snippet Edit form to Vue, this MR re-names the Snippet App into Snippet Show in order to prepare the ground for creating the Snippet Edit application in vue.

This is a ~backstage MR really, hence no CHANGELOG entry is provided.

NOTE: To click-test, you need to enable the :snippets_vue feature flag in the rails console.

Screenshots

Before	After

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

References #207463 (closed)