Enrolling can get disabled, requiring a service restart

[ChocolateLoverRaj]

Reproduce:

• Run rust-fp add test
• Either do a low-quality scan, or just exit the program in the middle of enrolling
• Then you can't add anything

Possible solutions:

• Allow users to cancel enroll operations that were created by their user, by doing rust-fp cancel-enroll or something
• Add a timeout (maybe 30s) to the enroll step so that even if a different user starts an enroll step that won't block other users from enrolling