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.
Fix up before filters and auth stuff.
Add user reference to levels, specifically those created with builder.
Fix seed task to only create rows once.
typo
Use shortened notation for consistancy.
Get rid of unnecessary current_user fallback. We require users to be logged in to get to this action.
Merge pull request #266 from code-dot-org/builder-new

Builder new