Update for Testnet2

[quentinlesceller]

Update the explorer for Testnet2.
Still need to compute the removed difficulty from total difficulty.

[hendi]

Thank you for the PR!

I don't think changing the 0001_initial.py migration manually is the way to go, I'd like to keep that and simply have Django create a new one: if you update the models.py then manage.py makemigrations blockchain does that automatically for you.

I also think we should keep the difficulty so that we can work with that data later, e.g. query for it or graph it. Thus the importer command should calculate and populate the DB with it.

[quentinlesceller]

Okay I'll open a new PR later today.