MM-11120 Adding setting to disable email invitations and rate limiting. (#9063)

* Adding setting to disable email invitations.

* Adding a setting and rate limiting for email invite sending.

* Modifying email rate limit to 20/user/hour

* Adding EnableEmailInvitations to client side config and command.

i18n/en.json

@@ -311,6 +311,10 @@
     "id": "api.command.invite_people.email_off",
     "translation": "Email has not been configured, no invite(s) sent"
   },
+  {
+    "id": "api.command.invite_people.email_invitations_off",
+    "translation": "Email invitations are disabled, no invite(s) sent"
+  },
   {
     "id": "api.command.invite_people.fail",
     "translation": "Encountered an error sending email invite(s)"
@@ -1614,6 +1618,10 @@
     "id": "api.team.invite_members.no_one.app_error",
     "translation": "No one to invite."
   },
+  {
+    "id": "api.team.invite_members.disabled.app_error",
+    "translation": "Email invitations are disabled."
+  },
   {
     "id": "api.team.is_team_creation_allowed.disabled.app_error",
     "translation": "Team creation has been disabled. Please ask your systems administrator for details."