Adjust registration form slightly to "disable" username changes

This is a short term adjustment while waiting for:

* #56
* https://tank.peermore.com/tanks/tank-notes/OauthIssues
Ajdust filedrop so it works on edit page too.

For the moment it thinks the dropped thing is an image. That will
change with a bit more tweaking.

This is a start at #55
Improve filedrop handling

This correctly deals with non binary mime types by putting
the binary array buffer directly to the server, instead of
building a JSON tiddler and putting that. This works in this
case because we don't need to worry about tags and the like.

Fixes #55