Add InvalidContentMessage and associated spec

What does this MR do?

• UX-wise, it provides a means of communication to the user that a non-markdown file was attempted to be used with the Static Site Editor (markdown files are a current limitation).
• Tech-wise, the StaticSiteEditor.vue component manages the dumb component's (InvalidContentMessage.vue) mounting when isSupportedContent = false.

Screenshots

isSupportedContent = true	isSupportedContent = false

The UI will not render until wired up by a future MR that has the planned StaticSiteEditor.vue parent component leveraging it. The original loose design guide for the above renders is found in this mockup.

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