Consider validate username before start marking cameras

[elKaZe]

I've registered and set my username, but when I started marking pins it showed an alert saying that the username wasn´t registered. I think it's would be nice to validate username at the moment it is set in the configuration.

[milo-trujillo]

Hmm, that's a good point. This would require extending the API so we can make a request that checks "Is this name valid", rather than the current behavior, which is to check as part of the camera-posting request. I can't think of any significant downsides to extending the API that way, though.

[milo-trujillo]

This is now implemented on the server and iOS client in the 1.4 branch, with an Android implementation coming soon. Unfortunately as a side-effect of #57 it is an extremely slow API call (> 2 minutes), and so the feature is effectively blocked until we have migrated enough users to the new login database format that we can discard the legacy database.