Show date of last posted message for each channel

As a server manager, I want to view a sortable list containing the date of the last message for each channel, so that I can identify stale channels in my server.

# How to access this data

1. [`TextChannel.lastMessage`](https://discord.js.org/#/docs/discord.js/stable/class/TextChannel?scrollTo=lastMessage) points to a text channel's last message (if it has one)
2. [`Message.createdAt`](https://discord.js.org/#/docs/discord.js/stable/class/Message?scrollTo=createdAt) is the `Date` that message was created

# How to store this data

We'll want to make this data available on channels stored in `db` so that it can later be accessed in `app`.

# How to display this data

An easy starting point for this would be to display it in a table with two columns:

- channel name ([`Channel.name`](https://discord.js.org/#/docs/discord.js/stable/class/TextChannel?scrollTo=name))
- last message date ([`Message.createdAt`](https://discord.js.org/#/docs/discord.js/stable/class/Message?scrollTo=createdAt))

These columns should be sortable (default to last message date desc?) to make it easier to find stale channels.