Extract base filter constant in Securitty Dashboard store's filters module

What does this MR do?

This extracts the base filter string "all" to a constant, and adds a utility function for checking whether an option is a base filter.

Part of the Instance Security Dashboard MVC.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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