Contract pausing implementation

[dusan-maksimovic]

• Add Pause action that can be executed by the Oversight committee to pause the contract.
• In all other actions check if the contract is paused and return error if it is.
• Support un-pausing through the governance by migrating to the same code ID (described in readme.md); migrate() function currently only un-pauses the contract.

[p-offtermatt]

There should probably be a query for whether the contract is paused

[p-offtermatt]

Nice. Left some comments

[dusan-maksimovic]

There should probably be a query for whether the contract is paused

We already have query for constants, so it would be a bit redundant.

[p-offtermatt]

You're right, that's good enough