In a PacketFence local account it's not possible to reset

[fdurand]

Describe the bug
When you edit an account and want to remove the value of "Login Remaining" ("Leave empty to allow unlimited logins.") and save then it return back to 0.
And 0 mean you don't have anymore attempt to use the account.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Users'
2. Edit a local user
3. Go in Password tab
4. Remove the value of Login Remaining
5. Save
6. The value is back to 0

Screenshots

Expected behavior
Support the value to be undefined or support negative values and update the explaination saying that a negative value is no account limit.