Record screen name changes

[jsha]

Users can change screen names on Twitter. Since we use user_id to keep track of them, this doesn't affect us much. Next time we update that TwitterUser, we will overwrite the old screen name with the new one. However, we should have an additional column, old_screen_names, that keeps a comma-separated list of old screen names. This may help resolve confusion if a user in a block list changes the screen name.

[cooljeanius]

This is another feature I wish Twitter would do natively, but until it does, having blocktogether do it instead would be nice