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.

api/context.go

@@ -282,7 +282,7 @@ func (c *Context) LogError(err *model.AppError) {
 	// filter out endless reconnects
 	if c.Path == "/api/v3/users/websocket" && err.StatusCode == 401 || err.Id == "web.check_browser_compatibility.app_error" {
 		c.LogDebug(err)
-	} else {
+	} else if err.Id != "api.post.create_post.town_square_read_only" {
 		l4g.Error(utils.TDefault("api.context.log.error"), c.Path, err.Where, err.StatusCode,
 			c.RequestId, c.Session.UserId, c.IpAddress, err.SystemMessage(utils.TDefault), err.DetailedError)
 	}