hagridctl regenerate
Introduces a "regenerate" command in hagridctl that will recreate all symlinks from currently "published" keys
This also introduces slight changes to the db structure:
- The symlinks are now under their own "links" directory. This allows having them on a different partition from the rest
- The "published" dir is shortened to "pub", to allow for inode-only symlinks on ext4 (again)
And fixes a bug I had introduced in !74 (merged) that would not correctly symlink some subkeys.