Resolve "Geo secondary help users not waste time on impossible operations."

What does this MR do?

It adds a global flash message for every page if you are currently on a secondary read-only geo node. To do this I added a new EE specific Application Controller, as I thought adding a new Partial to the Layout could have lead to the problem that it would also need restyling when flash messages get a new style. Before there was only a flash message when logging in.

Are there points in the code the reviewer needs to double check?

I am not sure how costly the Geo.primary_node check is, so maybe a instance variable to cache the value might be better? But that would need most probably extra code if you change the current status of the node, or ?

Why was this MR needed?

To make it obvious that any write operation will not work on a read-only node.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #2524 (closed)