Fix merge button disabled when MR is approved

What does this MR do?

• Fix merge button disabled when MR is approved
• For the non-API approvals json action, use new approval state entity when feature is enabled
• Ensure frontend approval_rules flag is scoped by project
• Allow widget to use new approved attribute instead of duplicating calculation logic at frontend.

What are the relevant issue numbers?

#9839 (closed) #1979 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?