Refactor common snippet controller methods

What does this MR do?

As discussed in #208786 (closed), Project and Personal snippet controllers have duplicated code. This MR refactors common snippet controller functionality into new snippet application controllers and concerns in order to reduce the duplication for the existing controllers, as well as new ones that are due to be created (e.g. a SnippetBlobController in #217775 (closed))

I've not added a changelog entry as this is just a developer facing refactor, and I've also not added any additional test coverage for existing functionality because the existing tests should be sufficient (I have added coverage for the new route helper, though).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Refs #208786 (closed)

Refs #217775 (closed)