#4755 Combining consecutive user join/leave system messages to single message and few other changes. (#5945)

fix 7 and 8

remove @ at "{username} joined the channel"

refactor and update test

webapp/i18n/en.json

@@ -1015,15 +1015,15 @@
   "analytics.team.title": "Team Statistics for {team}",
   "analytics.team.totalPosts": "Total Posts",
   "analytics.team.totalUsers": "Total Users",
-  "api.channel.add_member.added": "{addedUsername} added to the channel by {username}",
+  "api.channel.add_member.added": " {username} added {addedUsername} to the channel.",
   "api.channel.delete_channel.archived": "{username} has archived the channel.",
-  "api.channel.join_channel.post_and_forget": "{username} has joined the channel.",
-  "api.channel.leave.left": "{username} has left the channel.",
+  "api.channel.join_channel.post_and_forget": "{username} joined the channel.",
+  "api.channel.leave.left": "{username} left the channel.",
   "api.channel.post_update_channel_displayname_message_and_forget.updated_from": "{username} updated the channel display name from: {old} to: {new}",
   "api.channel.post_update_channel_header_message_and_forget.removed": "{username} removed the channel header (was: {old})",
   "api.channel.post_update_channel_header_message_and_forget.updated_from": "{username} updated the channel header from: {old} to: {new}",
   "api.channel.post_update_channel_header_message_and_forget.updated_to": "{username} updated the channel header to: {new}",
-  "api.channel.remove_member.removed": "{removedUsername} was removed from the channel",
+  "api.channel.remove_member.removed": "{removedUsername} was removed from the channel.",
   "app.channel.post_update_channel_purpose_message.removed": "{username} removed the channel purpose (was: {old})",
   "app.channel.post_update_channel_purpose_message.updated_from": "{username} updated the channel purpose from: {old} to: {new}",
   "app.channel.post_update_channel_purpose_message.updated_to": "{username} updated the channel purpose to: {new}",