Force push option, only displayed via opt-in in Preferences

See comments by Timothy Robb here:
https://answers.atlassian.com/questions/54469/how-do-i-perform-a-forced-push-push-f-from-sourcetree?page=1#comment-75151

Upshot is that for the minority of people that do use a workflow which requires force pushing, not supporting it is an issue. However, we don't want to offer it lightly, because it could be abused by people who don't understand the impact fully.

Proposed approach:

1. Option in Preferences to 'Allow force push'. Only when this option is enabled should the force push checkbox appear on the Push dialog
2. Using the force push option on the push dialog should still show a warning (with a 'Don't tell me again' checkbox)
3. The 'force' option should never be remembered in the push dialog