Make user login pruning configurable

[lordscarlet]

1. The time period for pruning the login history should be configurable.
2. There should be a way to indicate no pruning (Maybe through the value 0?)

Umbraco-CMS/src/Umbraco.Core/Persistence/Repositories/Implement/UserRepository.cs

Line 202 in 22df419

ClearLoginSessions(TimeSpan.FromDays(15));