Populate Static Site Editor Vuex Store with page source content

What does this MR do?

Creates a Vuex store action to load a page‘s source content into the Static Site Editor. This MR is part of a series of Merge Requests to deliver #212560 (closed). The other merge requests in this series are:

• Implement a service that fetches a page source content using GitLab API: !28598 (merged)
• Display page content in the Static Site Editor: !28472 (merged)

To test this work in your local environment, use the following integration MR: !28615 (closed)

See #212560 (closed) for more information. See the issue to obtain information about high-level design decisions and see user mockups.

Screenshots

This gif shows all the MRs working in conjunction:

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