disable button when 'ready for board review'

[t-keazirian]

Zenhub Link:

https://app.zenhub.com/workspaces/ksf-5f23520d91f7ee00134cbaf6/issues/the-difference-engine/ksf/302

Describe the problem being solved:

"Mark Stage As Complete" will be grayed out/disabled when application status is "Ready for Board Review"

Impacted areas in the application:

Mark Stage As Complete Modal

Describe the steps you took to test your changes:

Tested locally, showed to product team

If this ticket involves any UI or email changes, please provide a screenshot that shows the updated UI

List general components of the application that this PR will affect: MarkStageAsCompleteModal.js

PR checklist

• [X] I have linked the PR to a Zenhub ticket
• [X] I have checked for merge conflicts