Single source 'djdt' app name

[living180]

Avoid needing to keep multiple constants in sync by defining an APP_NAME constant in debug_toolbar.__init__ and referencing where it is needed.

[matthiask]

Hmm. We stopped single-sourcing the value in ff0b974 because of early importing. But yeah, this way around may work. (The old code imported urls in __init__.)

[living180]

Yeah, I hit #1581 as well, and looking at ff0b974, thought it would probably work to flip where the canonical version was stored. All tests passed for me and it didn't cause the #1581 issue to recur in my case.

[matthiask]

Thanks!