Rollout.flag_obj crashes if a flag gets renamed or removed

[Kos]

Repro:

• Create a flag
• Create a rollout with this flag
• Change the flag's id

Django admin now crashes on rollout list. Handle this gracefully.

[Kos]

A different but similar situation can happen (after #2) if:

• a Flag is changed to a TypedFlag & subject becomes mismatching
• a TypedFlag has changed types & subject becomes mismatching
• subject is changed & becomes mismatching