UC-145 Add FB disconnect button to prefs

RFCR @lizlux 

https://wikia-inc.atlassian.net/browse/UC-145
I would modify the `ident` string, similar to how it's done elsewhere in this file. Remove the part of this string that refers to where the `ident` string came from and add a string the refers to where it's going. Example: `ident.replace( 'mw-prefsection', 'container' )` or something like that. Currently it's a unique id that's being appropriated for a specific use, and if someone else came along and did the same thing, they would run into conflicts.