Use osm-auth instead of iD.OAuth.

It's a drop-in replacement and not seeing any issues with this switch
atm other than ones that were pre-existing - specifically the user shown
in the save dialog sometimes not being present or being out of date, but
the save still being correct.
Fix references to iD.OAuth
Remove sha1
Merge pull request #1184 from systemed/use-osm-auth

Use osm-auth instead of iD.OAuth.