First stab at builder.
Add create custom to levels
We can succesfully build an artist level, and return a link to it.
Move row insertion from migration to seed.
You can create a level, get a link, a use the link.
Use 'custom' blockly level instead of 'builder'.
Use constants for readability.

Use active record relation to improve readability.
Integrate all of Chris' feedback except for auth stuff.