Add upload file tab to the SSE image popup

What does this MR do?

This adds an upload file tab to the Static Site Editor insert image popup and displays the image in the WYSIWIG editor with a temporary URL.

This is the 1st iteration towards delivering #218529 (closed)

Note: this is behind the sse_image_uploads feature flag

To enable the feature:

1. Run rails c in the project root
2. Run Feature.enable(:sse_image_uploads) in the rails console
3. Refresh the browser

Screenshots

TODO

• Add upload image tab
• Display selected image in the WYSIWYG editor with a temporary url
• Add/update unit tests
• Add feature flag
• Code review

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Style guides