Add an application level flag to prevent editing in iCAT-X (e.g. during the backup)

[csnyulas]

Ideally this should be controlled also through the API, i.e. to set and reset such a flag that controls if user can edit or not the content through the UI or API.

[matthewhorridge]

The functionality has been implemented in the maintenance-redirect branch. This currently looks for environment variables to check for a redirect and this should be updated to look elsewhere

[alexsilaghi]

done in who-icatx/icatx-api-gateway#65
protegeproject/webprotege-backend-service#153
protegeproject/webprotege-gwt-api-gateway#41