Toggle ModMail

[iamshoXy]

This pull request adds two new commands:

• enable
• disable [reason]

When the bot is disabled, no more tickets can be created and the following message appears: "ModMail is currently disabled in {guild.name}. Reason: {reason}".
The reason for disabling is optional and the default reason is "No reason specified.".

When disabling the status column is set from None to the reason. If no reason is specified, it remains an empty string. As soon as the bot is enabled again, the field is set to None again.

[chamburr]

Thanks for your interest in contributing. A few major things regarding this pull request:

• For consistency, prefer a single command that toggles between on and off.
• Format of the bot response message when ModMail is disabled should match the ticket closed message.
• Database migration is missing.
• Command documentation on the web is also missing.

[iamshoXy]

Thank you for the feedback. Is a reason necessary or can I remove it?

[chamburr]

I think reason is rather useful for admins to provide the users with relevant information, for example, regarding why ModMail is disabled and how they could contact staff in the meantime.

[iamshoXy]

1. The command is now toggle and accepts a reason as before.
2. The embed formatting is now similar to the others.
3. Database migration added
4. Documentation added