[Mirror] Use only one webhook

[eplbot[bot]]

The bot should not create a new webhook each time a user sends a message. Instead it should simply keep one per channel and change its nickname a profile picture before sending a message. It would simplify the message deletion & modification timeout as well as the webhook will be always present in the channel

[Hokkaydo]

Additionally, threads and pins are not mirrored. Moreover, using the one-webhook method, we could consider showing reactions on messages

[Hokkaydo]

Implemented in #67