Fix overflow issues with monaco file editor

What does this MR do?

This MR fixes #229468 (closed) where an outer scroll container is conflicting with Monaco's built in scroll container. This outer scroll container is actually caused by using a <pre> tag for the root element for our editor.

This MR updates all #editor elements to use a <div> with a <pre> child, wrapping it's inner content, so that the preloaded state still looks nice with preformatted text.

Screenshots

Description	Before	After
Blob edit (with Feature.disable(:monaco_blobs))
Blob edit (with Feature.enable(:monaco_blobs))	20200727_overflow_before_blob_on	20200727_after_blob_on
Snippets edit (with Feature.disable(:snippets_edit_vue))
Snippets edit (with Feature.enable(:snippets_edit_vue))
Blob editor loading (shows for a split second)
Snippet editor loading (shows for a split second)

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

References

• #229468 (closed)