Compact the dashboard tree

### Description

I had imported accidentally twice the same test flow from Joe, to reproduce an issue. When restarting my dashboard I got this popup (*"You have multiple pages configured using the same url"*):


So now my dashboard is polluted with multiple pages and groups containing the same data.

I not really know what would be the most optimal way to solve this:
1. My first thought was that during import you should detect whether there exist already the same page/group with identical settings. However:
+ I assume you cannot easily adjust the import behaviour, because that is Node-RED stuff, not dashboard stuff.
+ You don't really know whether a user wants to merge pages and groups. Perhaps he wants to keep them side by side and just change the data afterwards, or remove only one of them later on.
2. So I "think" it would make more sense to add some button to the dashboard (or an action to Node-RED) that allows you to "compact" the dashboard tree. Which means to merge pages and groups that have identical settings.
3. Or that you can select multiple pages and groups in the sidebar, and that there is a join/merge button. Which could be used also for other purposes.

This is totally not urgent!!
Just registering it for the future, and that meanwhile people can add their thoughts in the discussion below...

### Have you provided an initial effort estimate for this issue?

I am no FlowFuse team member