Inaccurate Discord channel join/part messages relayed to Telegram

[prenetic]

This goes back to the original feature request (60). Currently, Discord join/part messages are being relayed, however depending on the Discord server's configuration they may be inaccurate.

For example, if someone joins a Discord server, but is not granted read permission to a channel that is linked between Discord and Telegram, then they have not actually joined that channel. Despite this, TediCross still relays to the Telegram side that someone has joined the Discord side of the chat. In actuality, this person needs to be granted read permission to the bridged channel before they will be able to even see it at all, let alone read any messages within.

Instead of relaying (potentially multiple) global Discord joins where many channels may be completely hidden from a new user's perspective, it would be more accurate to evaluate channel joins/parts based on read permission (or alternatively the per-channel member list) for each bridged room individually. This also has the added benefit of tracking and relaying channel parts when access to a given channel on the Discord side is revoked.

[Lulalaby]

Issue Check

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[prenetic]

As far as I'm aware, this has not been addressed.

[Lulalaby]

Oky. Just trying to catching up as I took over maintaining.