[Mod_menu] Make remove from menu text nicer

[DorienD]

Description

The text was the same as when you delete a resource, with a very scary text THIS CANNOT BE UNDONE. So i made it a little friendlier, since it can be undone because the resource is not deleted.

[mworrell]

Maybe we should make it scary only if the referred thing has:

• is_dependent set; and
• has only a single incoming edge (ie. the menu resource we are editing).

[mworrell]

Hmmm, that is then something that needs to be checked before we can show that confirm dialog. We will need an API call for that.

[mworrell]

Or should we add a property for the rsc model: is_dependent_with_single_referrer ? Kind of ugly.... though it describes the condition.