Design Management FE to FOSS Part 2/3: Init Design Mgmt. Vue app in CE

What does this MR do?

Moves initialization of the Design Management Vue app from EE to CE. This is part 2/3 in effort to move Design Management to FOSS (#212566 (closed)).

Important: Design Management will still be restricted to EE users (i.e. not usable in CE), making this a ~backstage change; this MR is part 2/3. The follow up MR to this one (!31309 (merged)) will complete the transition.

Additionally, we now use .js-design-management instead of #js-design-management to bootstrap the Vue app, to keep inline with our guidelines

Screenshots

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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