Switch redirectToOLFreeTrialUrl to calculate trial url

Sending unnecessary query params to OL seems like a footgun, so switch to
compromise that is more explicit but doesn't send unnecessary query params

services/web/app/views/layout.pug

@@ -98,10 +98,7 @@ html(itemscope, itemtype='http://schema.org/Product')
 
 		- if (settings.overleaf && settings.overleaf.host)
 			script.
-				window.freeTrialBaseUrl = '!{settings.overleaf.host}/users/trial'
-		- else
-			script.
-				window.freeTrialBaseUrl = '/user/subscription/new'
+				window.redirectToOLFreeTrialUrl = '!{settings.overleaf.host}/users/trial'
 
 	body
 		if(settings.recaptcha)

services/web/public/coffee/main/account-upgrade.coffee

@@ -11,7 +11,10 @@ define [
 			w = window.open()
 			go = () ->
 				ga?('send', 'event', 'subscription-funnel', 'upgraded-free-trial', source)
-				url = "#{window.freeTrialBaseUrl}?planCode=#{plan}&ssp=true"
+				if window.redirectToOLFreeTrialUrl?
+					url = window.redirectToOLFreeTrialUrl
+				else
+					url = "/user/subscription/new?planCode=#{plan}&ssp=true"
 					if couponCode?
 						url = "#{url}&cc=#{couponCode}"
 				$scope.startedFreeTrial = true