PLT-3131 Add slash command for sending email invites (#3218)

* Add slash command for sending email invites

* Add response for empty message and trim commas

* Check for @ in email addresses before sending

* Update invite people error message

i18n/en.json

@@ -339,6 +339,34 @@
     "id": "api.command.execute_command.start.app_error",
     "translation": "No command trigger found"
   },
+  {
+    "id": "api.command.invite_people.desc",
+    "translation": "Send an email invite to your Mattermost team"
+  },
+  {
+    "id": "api.command.invite_people.email_off",
+    "translation": "Email has not been configured, no invite(s) sent"
+  },
+  {
+    "id": "api.command.invite_people.fail",
+    "translation": "Encountered an error sending email invite(s)"
+  },
+  {
+    "id": "api.command.invite_people.hint",
+    "translation": "[name@domain.com ...]"
+  },
+  {
+    "id": "api.command.invite_people.name",
+    "translation": "invite_people"
+  },
+  {
+    "id": "api.command.invite_people.no_email",
+    "translation": "Please specify one or more valid email addresses"
+  },
+  {
+    "id": "api.command.invite_people.sent",
+    "translation": "Email invite(s) sent"
+  },
   {
     "id": "api.command.init.debug",
     "translation": "Initializing command api routes"