Submit changes in the Static Site Editor

What does this MR do?

Implements the action of submitting a change in the Static Site Editor. This Merge Request delivers part of the requirements described in #212561 (closed). See the issue for a full description of the requirements and to see design mockups.

NOTICE: This MR does not implement the service that connects with the GitLab REST API to save the changes in the editor. That will happen in another MR.

Screenshots

Can I test this Merge Request locally?

Yes, you can. Open the following MR in your local environment: http://localhost:3000/gitlab-org/gitlab-shell/-/sse/master/README.md

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
• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #212561 (closed)