Allow custom database name in config

[skybird23333]

Is your feature request related to a problem? Please elaborate.

Currently, when the bot connects to a MongoDB instance it uses the database name modmail_bot which is hard coded. This means a new MongoDB instance needs to be created for each instance of the bot.

Describe the solution you'd like

Allow the database name to be configured via .env, and have the following lines read from .env.

https://github.com/modmail-dev/modmail/blob/b21037cbd794a962569de6240c1d95c13009a64e/core/clients.py#L448

https://github.com/modmail-dev/modmail/blob/b21037cbd794a962569de6240c1d95c13009a64e/core/clients.py#L500

Does your solution involve any of the following?

• Logviewer
• New config option

Describe alternatives you've considered

Haven't thought of any, maybe to edit the source code directly(for users).

Who will this benefit?

People who need to host multiple modmail instances and don't want to make multiple MongoDB atlases/deploy multiple instances of the server for each bot.

Additional Information

No response

[raidensakura]

Duplicate of #3191