Supply a separate menu for use with subsites by default.

[rupertj]

While you can use any menu, it'd be nice if we supplied one for people to use.

• [ ] Add a menu by default when the module's installed.
• [ ] Ensure that existing content types can be added to it.
• [ ] Ensure that new content types can be added to it.

[markconroy]

Related issue: https://github.com/localgovdrupal/localgov_base/issues/526

[rupertj]

Closing this as we have a menu set up now. We still need to place the menu block for it automatically, but that should be a separate issue, I think.