Refactor snippet route helpers

What does this MR do?

In several places of the codebase we have conditions to check whether the snippet is a PersonalSnippet or a ProjectSnippet. Based on the type we will generate the proper route.

In this MR we overwrite existing named routes helpers to avoid that redundancy and also in preparation for secret snippets, where we will have to automatically add a query string param to every personal snippet route.

Refs #14201

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