Deal with changing 'screen_name' (twitter), 'login' (github) and maybe others

[zbynekwinkler]

Both twitter and github allow changing the easily recognizable string and both provide unmutable ids (that we store but not use for anything AFAIK). If we do not handle the change gracefully we should at least not blow up.

[chadwhitacre]

I think we're good on this?

[duckinator]

It also doesn't update actual names. Compare Bitbucket information on https://gittip.com/duckinator and https://bitbucket.org/duckinator.

[zbynekwinkler]

We will be good on this once #900 is implemented and only elsewhere platforms with immutable user ids are allowed for sign in (which I believe is part of #1369). There is no way to deal with platforms without immutable ids except dropping the attachment when user name changes :( (which will most likely be part of #900). Oh, and #487.

[Changaco]

Closing in favor of #900.