Remove ucf conffile conflict between grub-pc and grub-efi-{amd64,ia32}

Moves the ownership of /etc/default/grub from being split across all of the different GRUB "master" bootloader metapackages, and assigns ownership to grub-common instead. Also removes the Conflicts/Replaces that existed between grub-pc and grub-efi{amd64,ia32}. Note that grub-efi-amd64 and grub-efi-ia32 still conflict with each other, just neither of them conflict with grub-pc any longer.

Given a properly set up system, this allows both BIOS and UEFI bootloaders to be automatically updated when GRUB updates are released.