update mb-sessions, add unit tests

[4meta5]

What does it do?

• some small changes to improve readability i.e. replacing tuples (anonymous structs) with named structs
• move some constants to be in the module's Config instead of imported from the runtime (so still configured in the runtime, just in a more intuitive location)
• adds mock runtime

TODO (in-progress)

• unit test mb-session
• may change Treasury storage item and also remove the Vec<AccountId> for validators to use IterableStorageMap api instead to get the same functionality without the additional storage item

What important points reviewers should know?

I changed the crypto module in mb-sessions to only use sp_core::ecdsa as per the changes in #127 (I made this change in this commit: d732df2 ).

Is there something left for follow-up PRs?

I'll add unit tests to the other pallets as well. It is a good exercise for reviewing the runtime code.

What alternative implementations were considered?

Are there relevant PRs or issues in other repositories (Substrate, Polkadot, Frontier, Cumulus)?

What value does it bring to the blockchain users?

Adding unit tests to each pallet allow us to isolate and catch errors in the runtime instead of relying on integration testing or production to alert us after the fact.

Checklist

• Does it require a purge of the network?
• You bumped the runtime version if there are breaking changes in the runtime ?
• Does it require changes in documentation/tutorials ?

[tgmichel]

So cool to see this rise from the ashes, but we are not using this pallet right?

[4meta5]

@tgmichel Good point, I wasn't aware that mb-session and mb-core were not used.

Would it be better for me to start our chain governance pallets from scratch instead of working off this code? I think so, I guess I'll close this branch.