PLT-6484 Add /leave command to leave a channel (#6402)

* PLT-6484 Add /leave command to leave a channel

* Text changes requeted on review.

* PLT-6484 Display the right error message when trying to /leave town-square

* PLT-6484 Be able to execute /leave command in direct and group message channels with the same effect as clicking x

* PLT-6484 Refactor to create new leave_private_channel_modal.jsx

* PLT-6484 Remove previous leave private channel logic to use new leave_private_channel_modal.jsx

* Remove dot in command description. Change localized error when leaving Town square.

* disable /leave command in reply threads on the right-hand sidebar, since it is not obvious which channel you should leave

i18n/en.json

@@ -691,6 +691,30 @@
     "id": "api.command_join.success",
     "translation": "Joined channel."
   },
+  {
+    "id": "api.command_leave.desc",
+    "translation": "Leave the current channel"
+  },
+  {
+    "id": "api.command_leave.fail.app_error",
+    "translation": "An error occurred while leaving the channel."
+  },
+  {
+    "id": "api.command_leave.list.app_error",
+    "translation": "An error occurred while listing channels."
+  },
+  {
+    "id": "api.command_leave.missing.app_error",
+    "translation": "We couldn't find the channel."
+  },
+  {
+    "id": "api.command_leave.name",
+    "translation": "leave"
+  },
+  {
+    "id": "api.command_leave.success",
+    "translation": "Left the channel."
+  },
   {
     "id": "api.command_logout.desc",
     "translation": "Logout of Mattermost"