Add a Live Chat Event Block

It'd be interesting to create a block that can handle a live chat event (such as a [WordPress Meeting](https://make.wordpress.org/meetings/)) that would happen in three phases:

1. Before the event: Show the date and time when the event will happen, possibly with a countdown.
2. During the event: standard Chatrix functionality: display a Matrix room in single room mode.
3. After the event: display the transcript between the event start time and a defined end timestamp.

### Metadata
This block would need to have the following settings:
- Event start datetime
- Event end datetime (optional, might also be filled after the event is over)
- Access Token for the transcript (optional)

### Transition between the states

#### Before Event → During Event
When you are on the countdown page and the time arrives, it should load up Chatrix.

#### During Event → After Event
No transition. People should be able to continue chatting if they started off there.

### Details

#### After Event Screen
We'll need a way to get the transscript and for that we could use an access token to talk to the Matrix server and get the messages between the timestamps and cache them in a transient.

An option would be to add the ability to just paste the transcript using an innerBlock technique like the quote block uses.