Make "define/ inspect a type" form modal

[annedino4]

See #102 Because the "inspect type" page covers everything, it broke some students' mental flow, they were surprised to see the whole canvas disappeared and didn’t know what to do. After returning to the canvas, one student forgets what she was doing.

We could change it to a modal dialogue on the canvas, so it doesn't break the mental flow when defining/checking a type:

[georgefst]

We now render and edit typedefs on the canvas (#863, #968 etc.).