Resolve "Remove YAML front matter from editable content in the Static Site Editor"

What does this MR do?

• Prevents front matter from displaying in the SSE's (Toast UI Editor) WYSIWYG mode
• Still shows the edited source in its Markdown mode
• Manages updating the editor's v-model based on the mode (raw:'markdown'::body:'wysiwyg'

Due to the editor internally managing the content, we need to manually update the v-model on mode changes. The parseSourceFile service's returned instance works in tandem with the edit_area.vue to achieve this.

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