R1:84cf49387984

Allow "Manage" to be the default menu item for projects

Summary:
Fixes T13033. Currently (prior to D18836) all the default items for projects can be hidden. When this occurs, the main project page fatals.

If we fix the fatal narrowly (don't try to call `null->getBuiltinKey()` when `$default` is `null`), it would 404, which is a little better but not by much.

After D18836 you can't hide "Profile", which is pretty sensible, and effectively fixes this. This change doubles down: let "Manage" be a default…