Allow for configurable ChatManager.CommonMention

[ZeWaka]

Is your feature request related to a problem? Please describe.
Not exactly.

Describe the solution you'd like
Title, I would like to be able to change the chat prefix from !tgs to an arbitrary string. This could be strings such as:

• .tgs
• ]server
• ;

Describe alternatives you've considered
There are none that I can think of.

Additional context
no other context - tried implementing this and it was a pain to get yml appsettings propagating down to the discordprovider from the chatmanager level

[Cyberboss]

Duplicate of #1237