Add command to reset repo authentication on Keychain Access

[carlosefonseca]

When using a repo with authentication, Fork prompts for it and saves on Keychain.

When that authentication expires, git returns errors and we have to open Keychain Access and delete the corresponding entry. This is specially annoying because Keychain Access doesn't allow deletion of entries when searching.

I think it would be useful if Fork had some option or button, maybe in the Repository Settings, that would delete the corresponding "Fork.servername" Keychain entry. This would make it easier to use and explain to coworkers.

Thanks!