[New Tutorial] Advanced Runtime Upgrade + Storage Migration

[danforbes]

Create a new tutorial or add a new section to the existing tutorial on runtime upgrades. The new material should demonstrate the details of storage migrations as well as strategies for testing them.

Relates to #580 and #750.

[apopiak]

Minimum steps:

1. run original node template
2. change Something get(fn something): Option<u32>; storage to store SomeStruct (and define SomeStruct).
3. add storage migration function like here except putting it in a separate migration module and migrate_to_struct function. (might be missing some logging)
4. compile the new runtime and perform an upgrade

[apopiak]

We could also think about introducing tests. Like a unit test for the migration function. And/Or an integration test via Halva or the substrate-test-runner.

[nuke-web3]

Closing in favor of #939 to consolidate issues here.

With respect to unit tests, we may be able to solve #638 with this one as well...?