Update hook to migrate menu / breadcrumb reaction settings to new format.

Having to upgrade the context settings manually in every case is very time consuming, especially for sites with large menus where the default option is not set cleanly - if, say, the menu options were not built properly.

Attached update script automatically migrates the menu and breadcrumb reactions where possible and required using the same path as the default option from legacy configuration.
Only caveat I can see is that sometimes the wrong menu could be selected - I'm not sure how this could break legacy configs further, and only having to change some contexts seems to me to be an improvement over reconfiguring all of them to get the new format.