`Mark Broken` and `Mark Fixed` permissions are too restrictive and inconsistent

While working on LP:#1889026 and LP:#1994899, I found `Mark Broken` and `Mark Fixed` permissions unnecessary restrictive and inconsistent:

- `Mark Broken` is actionable only for machines owned by some user. This means that freshly-commissioned machines in `Ready` state cannot be marked as broken.

- `Mark Broken` is only available to the user that has ownership over the machine. While it seems logical, it also means that the action is not available to the MAAS administrator if the machine is owned by a simple user (as noted in https://bugs.launchpad.net/maas/+bug/1811234). This limitation also ignores RBAC rules permitting marking machines broken.

- `Mark Fixed` requires `NodePermission.admin` while `Mark Broken` requires `NodePermission.edit`. Users locking machines that belong to them will not be able to mark machine as fixed on their own, requiring administrator to intervene.

While all three issues have pretty straightforward fixes to them (and there is an MP for the first two already), applying them might cause non-obvious security implications. Further analysis is required.