Add /help slash command (#6650)

* Add /help slash command

* Fix copyright year

* go fmt

* Add help command test at api4

* Fix jasonblais's issue

* Update test code

* Fix unit test issue - keep config unchanged

i18n/en.json

@@ -871,6 +871,14 @@
     "id": "api.command_shrug.name",
     "translation": "shrug"
   },
+  {
+    "id": "api.command_help.desc",
+    "translation": "Open the Mattermost help page"
+  },
+  {
+    "id": "api.command_help.name",
+    "translation": "help"
+  },
   {
     "id": "api.compliance.init.debug",
     "translation": "Initializing compliance API routes"