Before starting work on this ticket, please check the GitHub help wanted issue first to see if anyone has started on it: https://github.com/mattermost/platform/issues/4755

A) Join/leave messages should:

1. Never trigger a desktop notification unless you are mentioned by someone else (eg @lindsay added @asaad to the channel)

2. Never trigger an “Unread” bolded channel name unless you are mentioned by someone else (eg @lindsay added @asaad to the channel)

B) Update the grammar for the messages:

1. Switch the joined a channel message to say:
   @lindsay joined the channel.

2. Switch the left a channel message to say:
   @lindsay left the channel.

3. Switch the added to a channel message to say:
   @lindsay added @asaad to the channel.

C) Combine Consecutive join/leave messages

System messages related to joining, leaving, and adding/removing people from channels are combined into one post if they happen one after another, without any non-action messages in between. The messages are simply appended to the first message, and appear in the order that they occur.

Example:

@lindsay joined the channel. @lindsay added @asaad to the channel. @lindsay left the channel. @jason joined the channel.