Refactor file upload in Personal Snippets

What does this MR do?

This MR moves the method move_temporary_files from the SnippetsController to the Snippets::CreateService. Why? Because we are implementing the creation/edition workflows in VUE and the existing controller actions won't be called. Therefore, if we want this logic, we have to move it to the service that is called from both the controllers, API, GraphQL, and VUE.

The workflow related to move_temporary_files only applies to personals snippets, not project snippets.

Refs #217716 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry
• [ ] Documentation ([if required](https://docs.gitlab.com/ee/development/documentation/workflow.html#when-documentation-is-required))
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content