Admin-updatable Welcome Message

[Tobio89]

I want to add a feature where admin can edit the welcome message for the bot.
The bot should read the welcome message from the DB before posting it.

The admin will post the welcome message as a regular message, and copy that message's ID.
The command takes the message's ID and the bot will find the message, copy the content, and save it to the DB.
It needs to maintain the markdown structure.

The bot should store the time the welcome message was updated.
Rather than loading the message from DB each time, it can check the updated time, and then reload the message only if it's been changed.

We will probably need a command to test the welcome message, so that we can make sure it's correct.