Use VueApollo to load source content in the Static Site Editor

What does this MR do?

1. It replaces the Vuex mechanism used to load the source content with a VueApollo query.
2. Since VueApollo won’t approach state management to track changes in the editor, this MR implements tracking changes in the edit area component.
3. As an effect of point number 2, we’ve moved the rich content editor and publish toolbar components inside edit area. Those components depend on tracking changes in the source content.

bonus: I wrapped the skeleton loader config into a separate component. I’m happy to move that change into a separate MR.

This MR is part of the effort to adopt GraphQL in the Static Site Editor. You can see more details of the frontend efforts at #216186 (closed).

To understand the whole effort, see &3183 (closed)

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

Closes #216186 (closed)