Send a batch of dispatch calls.

May be called from any origin except `None`.

- `calls`: The calls to be dispatched from the same origin. The number of call must not
  exceed the constant: `batched_calls_limit` (available in constant metadata).

If origin is root then the calls are dispatched without checking origin filter. (This
includes bypassing `frame_system::Config::BaseCallFilter`).

## Complexity
- O(C) where C is the number of calls to be batched.

This will return `Ok` in all circumstances. To determine the success of the batch, an
event is deposited. If a call failed and the batch was interrupted, then the
`BatchInterrupted` event is deposited, along with the number of successful calls made
and the error of the failed call. If all were successful, then the `BatchCompleted`
event is deposited.

- There is a picture of a website called polkassembly.
- The website is talking about something called a referendum.
- The referendum is like a vote where people can decide on something.
- The website is making some changes to how the referendum works.
- They want people to give their opinions on the changes.

Referenda #251