Make the registry accept themes without main.js

[mackenza]

Theme extensions require an empty main.js in order to be uploaded to the registry. I think there had been talk to remove that restriction.

[dangoor]

Ouch. You're right, we need to fix that.

[dangoor]

Nominating for fixing in release 0.43.

[dangoor]

Reviewed medium priority in Extension Registry

[dangoor]

Not a blocker for 0.43, but should be in 1.0