Config to make town square read only (#7140)

* Be able to make Town Square read-only (Disable typing messages for non admins).

* Do not emit UserTypingEvent when TownSquareIsReadOnly and is Town Square.

* Add unit tests for TownSquareIsReadOnly config value and logic.

* Add TownSquareIsReadOnly to System console>Policy. Added Telemetry.

* Add control for TownSquareIsReadOnly=true only for License Enterprise Edition E10 & E20.

* Update en.json

* Update en.json

* Update policy_settings.jsx

* Change config value from TownSquareIsReadOnly to ExperimentalTownSquareIsReadOnly.

* Refactored to simplify. Avoid code repeat and multiple db calls.

config/default.json

@@ -74,7 +74,8 @@
         "UserStatusAwayTimeout": 300,
         "MaxChannelsPerTeam": 2000,
         "MaxNotificationsPerChannel": 1000,
-        "TeammateNameDisplay": "username"
+        "TeammateNameDisplay": "username",
+        "ExperimentalTownSquareIsReadOnly": false
     },
     "ClientRequirements": {
         "AndroidLatestVersion": "",