Define constants for routes within Design Management

What does this MR do?

Defines constants for route names within Design Management

Why?

To promote consistency when using route names throughout Design Management.

What's in this MR?

This MR does 4 things:

• moves ee/app/assets/javascripts/design_management/router.js => ee/app/assets/javascripts/design_management/router/index.js
• adds ee/app/assets/javascripts/design_management/router/routes.js, which exports the array of routes to use in VueRouter
• adds ee/app/assets/javascripts/design_management/router/constants.js, which defines and exports all route names used in Design Management
• updates all code somehow using $router to use named routes, the names of which are defined in routes/constants

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

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